Preliminary Note.

In 1784, the attention of the French Government having been directed to the desirability of protecting the powder magazines of the kingdom from damage by lightning by the employment of conductors, a system of construction was proposed by two officers of the Engineers and Artillery. This system was referred by the Minister of War to the Academy of Sciences for consideration and report. From time to time subsequently other proposals of a like nature, and other inventions and improvements in the construction of lightning rods were considered by the Academy, and reported upon by various Committees.

At the request of the Conference, I have endeavoured, in the following pages, to give in as condensed a form as possible an accurate abstract of their contents, and to avoid, in all cases, the expression of any opinion, either adverse or concurrent, upon the principles or suggestions contained in them.

REPORT made to the ACADEMY OF SCIENCES, by Franklin, Leroy, Coulomb, de la Place, and Rochon.

Certain proposals for erecting lightning rods for protecting the powder magazines at Marseilles having been submitted to the Academy for their opinion, a committee, consisting of the above-named, was appointed to examine and report.

They begin by enunciating the theory which should regulate the erection of conductors, and they lay down the following rules:—

1. The extent of the building should first be ascertained to decide whether one or more conductors should be used. Electrical experiments had not yet made known anything of the extent to which the action of the point of the conductor reached. But since buildings had been supplied with conductors many observations had shown that those parts of them which were more than 45 feet French (48 English) from the point of the conductor had been struck by lightning.

2. When there are many points or arrows on the building they should be connected together and also connected with all parts of the roof which are covered with lead, and also connected with the weathercocks or ornamental metal points so as to form one metallic system with the conducting bars.

3. It is not less important that these bars should be thoroughly joined together; for a solution of continuity in them produces a resistance to the passage of the electricity according to the extent of their separation.

4. It is necessary that the bars should communicate thoroughly with moist earth or, better still, with water.

As to the height of the points they should be at least 12 or 15 feet (13 to 16 feet English), or even more if the building is a large one. It is certain that the higher they are the wider the extent of their action. They should be 2 inches (2·2 English) square at the base and greater in proportion as their height exceeds 15 feet (16 English). If the conducting bars are 8 or 10 lines (or, say 1 inch) square, it will be more than enough. No case had occurred in which iron bars of this size had been in any way damaged or altered by the passage of lightning.

The reporters then proceed to examine the two proposals for protecting the powder magazines at Marseilles, sent in by M. Ravel de Puy Contal and M. Pierron. They were both for the same building which was 31 toises long and 8 toises wide (about 198 by 51 English feet). The first provided for the erection of three points on the ridge of the roof, and of four others, one at each angle of the building; the second had also three points on the ridge, but the other four were alternated on the two sides of the roof, and iron bars were carried all along and connected with all the points. The manner in which the terminals were fastened to the roof and the conducting bars fastened together and led to the water was the same in each proposal.

The reporters remark concerning the second that the conducting bars laid horizontally along the roof would involve a great and unnecessary expense, but the points should be retained, only instead of placing them alternately they should be set up so that each of them was half way between the middle and the end of the roof, and instead of connecting these points by bars along the length of the roof, they should be connected with the one connecting the three points on the ridge by bars joining it perpendicularly.

As to the method proposed for joining the several parts together the reporters cannot help thinking that in their desire to make thoroughly good connections MM. Pierron and de Ravel had proposed a plan involving too much difficulty and superfluous expense [It seems to have been proposed to screw the bars into each other], and they recommend instead of this to make at the base of each point, immediately above its insertion into the roof, a circular flange about 2 inches in diameter and 2 lines thick, with a hole half an inch in diameter in the middle and at the ends of each conducting bar to make a similar flange and to bolt the flanges together with a sheet of lead between them. Crutches should be fixed on the roof to carry the conducting bars. The points should be fixed three on the ridge and two on each side of the roof half way between the point in the middle and that at each end. These four should be connected with the conducting rods running along the ridge and should overtop the ridge by at least 6 feet (6 feet 5 inches English). By this arrangement all parts of the roof would be well protected.

The reporters highly approve of the way in which the conducting bars are connected with water by being led into the sea, but if at the other end of the building there is sufficient earth on the surface, and the soil is not entirely rock the conductor from the point placed at that end might be led into it. It is recommended that the points of copper should be screwed to the terminals for convenience of removal when necessary.

REPORT made to the NATIONAL INSTITUTE, by Leroy, La Place, and Coulomb, on a Lightning Rod for Powder Magazines proposed by Regnier.

The reporters think it desirable to make some general observations on lightning rods, the rather that it appears that some persons have had fears as to the certainty of their effect.

It is impossible to reject the theory upon which Franklin had proceeded in providing lightning rods for the purpose of protecting buildings from damage by lightning. Still, as the theory needed to be confirmed by facts, it might at first have been doubted whether the lightning rods were really effectual; but now that observation, and experiment had proved the truth of the theory there was no longer any room to question their utility. It may even be remarked that observations had not only proved that they were effective when well constructed, but that they conducted the lightning down without accidents, even when they had some defects, which might have caused one to doubt their efficacy. The defects alluded to were a blunted point and a break in the continuity of the conductor. With reference to these two cases observations have shown—1st. That although the points have been blunted, they still attract the lightning from the clouds to themselves in preference to the surrounding objects. 2nd. That although the several parts of the conductor are not thoroughly joined together, the lightning will still, if the break be not too considerable, pass along the conductor without accident.

In support of the first proposition they quote the observations of Doctor Rittenhouse, of Philadelphia, who had examined several of the points in that city, and had found them melted, showing clearly that they had been struck by lightning, and probably more than once, as it had been shown by many observations that where, from local circumstances (not then fully ascertained,) lightning had struck in certain places or on certain buildings, it was not uncommon to see it strike again; and a number of observations of a different sort had shown that lightning was attracted by metals on buildings even when they were but slightly pointed, such as tin weathercocks, or iron crosses, and even plain sheets of iron.

One of the most striking examples in support of the second proposition, was the case of an American ship, reported in the Phil. Trans. for 1770. During the night, in the midst of a storm, the crew reported that there was a stream of fire in the rigging, just above the middle of the lightning conductor. The captain saw a stream of fire, sometimes in sparks, and sometimes only a steady light; and on examining the conductor next morning, found that one of the links of its chain was broken. Fortunately the two pieces, being kept in place by the fastening to the shrouds, were only about three quarters of an inch apart. These two broken ends formed a sort of points, and on its passage between them the lightning had become visible. But this was all; no shock was felt, nor anything which caused any suspicion that the fracture of the conductor had in any way hindered the passage of the lightning. Franklin also had shown by experiment that in a lightning rod where the upper end was only connected with the part entering the ground by a very fine brass wire, although the wire was melted by the passage of the lightning, it still was conducted from top to bottom without any damage to the house; and in other instances metallic wires, though partly melted by the lightning, had still served as conductors. But it is not contended from these examples that a very exact and continuous connection of all the different parts should be dispensed with.

The lightning rod proposed by Regnier consisted of a piece of wood, coated with resin, rising 2 metres (6 feet 7 inches) above the roof, and having fixed on its top a sort of inverted funnel of copper, at the upper end of which was fixed the point. To the lower edge of the funnel were fastened ropes formed of twenty-seven annealed iron wires well bound together, which were, at a suitable distance, connected with iron bars, fastened to masts, and leading to moist earth. The point had a small piece of platinum at its upper end.

The reporters observe that the wooden support may be employed by way of extra precaution, though there was no known instance of lightning leaving metal for wood; but it should be strong enough to resist the wind. They approve of the method proposed for connecting the point with the metal bars, metallic ropes being very suitable for this purpose, and keeping them well away from the building was quite right; but they add that the metallic bars should not only communicate with moist earth, but also with water in wells or otherwise.

INSTRUCTIONS for erecting Lightning Rods for Powder Magazines, adopted by the Fortifications Committee.

A lightning rod is an electrical conductor terminating in a point and carried down to the common receiver. It may be regarded as a metallic tree, and divided into (1) the upper terminal, (2) the trunk, and (3) the roots.

1. The upper terminal is a very pointed, conical or pyramidal spike of metal having a base 3 or 4 centimetres (1½ inches) in radius. The point is of gold or platinum, soldered to a copper rod 1 or 2 metres long (3 feet 3 inches to 6 feet 7 inches). This rod is joined to the rest of the upper terminal, which is of iron, either by solder, a screw, or a pin. It is important that all the parts of the upper terminal should be joined with care so as to prevent fracture; at the bottom of the terminal are several feet by which it can be leaded to the vault or bolted to the framing of the roof. Several devices for giving some play to the terminal so as to diminish the effect of vibration have been proposed, but it is better to make the terminal strong enough to resist. At the bottom of the terminal is joined the piece connecting with the conductor; this ought to be very complete and continuous, especially at the point of junction with the terminal. Frequently the terminal is enlarged at this point to facilitate the passage of the lightning. To preserve the terminal from rust it is sometimes gilded—it has been proposed to tin it—more frequently it is merely painted; experience shows that this is sufficient. Instead of making the whole terminal conical or pyramidal, a square bar of iron, finished with a point of copper tipped with gold or platinum, is sometimes used. This plan may usually be adopted without danger, but they are more liable to be broken or bent by vibration.

2. The trunk or conductor is made of iron bars 13 to 20 millimetres square (½ to ¾ inch) notched at the ends and bolted together with a plate of lead between the two (Fig. 1). For powder magazines a bar of 27 millimetres (1 inch) square is recommended. They follow the outline of the roof, cornice, and wall, and each bar is fixed by a half collar (Fig. 2) or cramp placed in the middle of the bar or as far as possible from the junction of two bars. Instead of the iron bars ropes of copper or iron wire, or even of hemp, may be used; these last may be used provisionally, but for permanent conductors they have no advantage either in economy or conductivity. The copper rope conducts the lightning better, but its smaller size and cylindrical form, by diminishing its absolute and relative surface, counterbalances its superior conductivity. The great and real advantage of metallic, and especially of copper ropes, is in their continuity and their flexibility. The conductor is led down to the surface of the ground where it is bent and led parallel to the surface towards a pit full of water, or deep enough to allow the end of the conductor to rest in damp earth; from 2 metres (6 feet 6 inches) above the ground to the pits the conductor is enclosed in a channel or trough like the fuse of a mine, the object of this is to protect the conductor from the dampness of the soil and from contacts. These would be unimportant so long as there is a perfect connection between the point of the conductor and the common reservoir, but this continuity may be destroyed by degradation of the conductor, and it is chiefly at the joints that this discontinuity is to be feared. When the conductor has to be buried it should be in an oaken trough, well put together and tarred or charred or surrounded by powdered charcoal so that the metal cannot be rusted by infiltrations or humidity; in some soils it is better to make the subterranean part of the conductor of lead, taking care by increasing the surface to make up for its inferior conductivity. Sometimes water pipes may be made use of, but only when they serve to lead water away and when they terminate in an isolated reservoir. It is important to lead the conductor far away from water pipes carrying water to public fountains or into the interior of houses.

3. If the conductor leads to a well full of water the roots (Fig. 3) need not be more than a few spindles terminated in points and long enough to be always immersed. When the conductor only leads to a bed of earth it is supplied with a system of roots (Fig. 4), having for its object the multiplication of the points for the escape of the lightning, and these are increased in number according as the soil is a less good conductor. The pits should be some distance from the foundations of the building, so that the lightning may not damage them, and it is important, by all possible means, to increase the natural humidity of the soil. When the wells cannot be closed it is necessary that the conductor should be insulated and plunged deeply in the water for fear that the communication of the electricity to the well-chains or pump-rods might cause accidents or alarm. After some other instructions it is added that the dispersal of the electricity in the common reservoir is, next to the continuity of the conductor, that which most deserves the attention of the physicist and the engineer.

It has been remarked that a point extended its sphere of activity as far as 10 metres (32 feet 9 inches), that beyond this distance its effect became less sensible, and that when the points were too near together they neutralised one another. So upon a building of a given size it is necessary to set up so many that all parts shall be covered by their spheres of attraction, which should meet and not overlap each other. Lightning in passing from a cloud to the earth does not always take a vertical direction, it sometimes follows the path of the rain drops, which is inclined by the wind, so when a magazine is very lofty, or on an elevated spot it is not useless to fix horizontal or inclined points on the gables or angles. In some places the magazines are dominated by other buildings; in these cases the neighbouring buildings should be protected, or the magazines should have horizontal points towards them. If the ramparts dominate the magazine it will be prudent to set upon them a lightning rod on a mast. Trees are only struck by lightning because their tops serve as points but their trunks are bad conductors, hence it is prudent not to have plantations, especially of lofty trees near magazines. However many points may be set up on a magazine they should all be connected together, and all joined to the principal conductor, and it would be well to have more than one principal conductor so that if one loses its continuity the lightning may have a path by the other. Stone, wood, and gunpowder are bad conductors, and pieces of metal may without danger be used in the inside of magazines, provided they are connected with the principal conductor by branch conductors of suitable size: still it is prudent to keep the metal outside.

Reference is then made to “Regnier’s System of Lightning Rods,” Appendix F., p. 53, which is thought to be much too expensive.

REPORT on the foregoing Instructions made by La Place, Rochon, Charles, Montgolfier, and Gay Lussac to the National Institute.

The reporters say that experience has taught that the point of a lightning rod 4 or 5 metres (13 to 16? feet) does not effectually protect a space round it greater than one having a radius of 10 to 12 metres (32¾ to 39¼ feet). That when there are points or considerable masses of metal on a building having a lightning rod it is absolutely necessary to connect them by branches with the principal conductor. That it is not less important that the metallic bars should be thoroughly well connected together so that the electricity may find no resistance in its path from the point to the common receiver. And lastly, that it is necessary that the conductor should have a perfect communication with moist earth, or better, with water. They then proceed to discuss the instructions, or that part of them which relates to the construction of the lightning rods. They recommend the use of gilded copper points, notwithstanding the doubt concerning them which had been raised in consequence of their deterioration by oxidation, and their being blunted by lightning. They say that experience has shown that an iron rod 20 millimetres (·8 inch) square is more than sufficient to carry the most violent discharge of lightning, and that it is consequently needless to make them larger, as recommended in the Instructions; that it is only at the joints that there is any cause for fear because, in spite of the insertion of the piece of lead, the contact is not perfect; that it would be easy by enlarging the bars at their junctions to increase the number of points of contact, and by lengthening the bars to make fewer joints. That in this respect the use of iron wire ropes would be very advantageous, but they fear that the ropes would be easily destroyed, and that the use of copper wire rope instead of iron would be too expensive.

When the conductor reaches the ground too much care cannot be exercised in making a free communication between it and the soil. It is upon this that its good effect principally depends, for houses have been struck although provided with a conductor, because it only communicated with a very dry soil. M. Patterson, of Philadelphia, in the fourth volume of the American Phil. Trans., has published a means of making a good contact which seems useful. He proposes to lay the conductor in a bed of galena worked into a paste with melted sulphur. The galena is a good conductor, and would have the advantage of protecting the iron from the damp. He has also proposed a simple means of providing for the easy dispersion of the electric fluid in cases where the soil is not very damp, which consists in making a hole in the ground and filling it with charcoal, into which the conductor is plunged. But M. Guyton used the conducting power of charcoal for this purpose more than thirty years ago, and it has been applied in many ways. Charcoal, like galena, is a good conductor, and this property renders its employment desirable in cases where the soil is dry.

Upon the proposal to fix inclined or horizontal points they think that vertical points will suffice; and with reference to the Regnier system, they remark that it would certainly be very expensive, and that it would not be necessary to adopt it until the usual system had been found insufficient.

After some theoretical remarks the Committee describe the conductor they recommend, giving the name of tige (upper terminal) to the part rising into the air above the roof, and that of conductor to that part extending from the upper terminal to the ground. The upper terminal is a square or round bar of iron tapering from base to summit. If from 7 to 9 metres (23 feet to 29 feet 6 inches) high, which is the smallest height to be used on large buildings, it should be 54 to 60 millimetres (2·1 to 2·3 inches) square or diameter at the base, if 10 metres (32 feet 9 inches) high, it should be 63 millimetres (2·5 inches). About fifty-five centimetres (1 foot 9½ inches) of the upper end is cut off and replaced by a point of copper either gilded at the end or tipped with a little piece of platinum. At the lower end of the terminal (A), 8 centimetres (3·15 inches) above the roof, is fixed a base (B) to throw off the rain which would run down the terminal, and above this base the terminal is clasped by a collar (C), as shown in the drawing, to which is bolted the conductor (D). The engraving shows the modification of the arrangement as adapted to both round and square terminals. The conductor is a bar of iron 15 to 20 millimetres (·59 to ·79 inches) square, joined firmly to the upper terminal by bolting it tightly between the two ears of the collar. The best way of joining the bars together is shown in figure 1, p. 55. It is to be held up at a distance of 12 to 15 centimetres (4·7 to 5·9 inches) from the roof by crutches, and to be kept at a like distance from the walls of the building. At 50 or 55 centimetres (19·6 to 21·6 inches) below the surface it is turned away perpendicularly from the wall for a distance of 4 or 5 metres (13 feet 1 inch to 16 feet 5 inches) if it does not sooner meet with water. To avoid rusting the rod is carried in a trench filled with charcoal, and then turned down a well so as to have at least 65 centimetres (25·7 inches) in the water when at its lowest level, where it terminates in three or four branches to facilitate the exit of the electricity from the conductor.

If there is no well convenient, a pit should be made 13 to 16 centimetres (5·1 to 6·3 inches) in diameter, and 3 to 5 metres (9 feet 10 inches to 16 feet 4 inches) deep, down the middle of which the conductor should be led and the hole filled with charcoal tightly rammed. As the iron bars forming the conductor are not easily bent to follow the lines of the building a metallic rope may be used. It is made of four strands, each composed of 15 iron wires, and forming a rope of 16 or 18 millimetres (·62 to ·7 inches) in diameter. Each strand is tarred separately, and the whole also well tarred when put together. It is attached to the upper terminal in the same way as the bars by pinching between the ears of the collar (c). At 2 metres (6 feet 7 inches) above the ground it is joined to the bars which form the earth connection by being pinned into a socket formed at the end of the first bar. Ropes of copper or brass wire may be used, and they need not be more than 16 millimetres (·62 inches) in diameter.

It is necessary to connect any considerable metallic masses (lead roofs, metal gutters, or tie rods) with the conductor, because if this be not done, and the conductor be broken, or have a bad earth connection, the lightning may leave the conductor for the metallic mass.

Modifications of this form of conductor for use on churches, ships, and powder magazines (for the latter carrying the conductors on masts is recommended) are then described.

The report says that the terminal of a conductor protects efficiently a circular space round its base, having a radius equal to twice its height; but that it is prudent to estimate that a conductor on a church spire only protects a circle having a radius equal to the height of the conductor.

The conductor should go the shortest way to earth. It should be on the side most exposed to the weather, especially on spires.

Second Part, 18th December, 1854.
Prepared by a Committee consisting of MM. Becquerel, Babinet, Duhamel, Despretz, Cagnard de Latour, and Pouillet.

Notwithstanding the considerable advance in knowledge since 1823, the instructions of that date have no need to be altered, at least in their essential principles; but the methods of construction of buildings having materially altered, and metal having largely replaced wood and stone, buildings had, so to speak, become metallic masses, which would have incomparably greater attraction for thunder clouds. The Palais d’industrie in the Champs Elysees, for example, nearly 3 hectares (7·4 acres) in extent, and 40 metres (131 feet) in height had everywhere enormous masses of iron, brass, and zinc.

The company undertaking the building had sought the advice of the Academy as to the means to be employed to protect it from lightning, and it had been found necessary to revise the instructions of 1823, in order to introduce such modifications as were necessary.

Quoting the passage referring to the connecting of metallic masses with the conductor, the Committee think that the time had come to enter into fuller details on this point.

Formerly the use of metal was almost restricted to ridges, gutters, and tie rods; now metal was used everywhere, and what is important, in large surfaces and great masses; and this new system realised on a large scale the first objection to lightning rods—it attracts the lightning.

When this objection was applied to lightning rods, it had only the appearance of truth, but when applied to the masses of metal then used in buildings, it was not only specious, but true, and founded upon well established laws; these buildings do attract the lightning, and render its effects more disastrous.

In the case of two buildings alike in size and shape, situated on the same soil, one made of wood and stone as formerly, the other with much metal as now, and both without lightning rods—if the conditions are such that the lightning must discharge itself, it will always strike the latter, and never the former; in the same way as on bringing to the conductor of an electrical machine a ball of wood or stone, and one of metal, it is always the latter which will receive the spark. Lightning rods, therefore, are so much the more indispensable as the buildings contain greater surfaces and greater masses of metal.

The nature of the soil must be taken into account, as well as the buildings and other objects upon it. A dry soil, with a subsoil of dry sand, chalk, or granite, does not attract the lightning, because it is a bad conductor. Unless when accidentally wetted the buildings on it participate to some extent in this immunity, at least if they are not built in the modern style, and are not very large. But if there are at a moderate depth underneath this dry ground, large metallic veins, vast caverns, sheets of water, or only abundant springs—these will attract the lightning, which will destroy everything in its path unless protected. If the wet or metallic strata are very deep, the danger of an explosion is diminished by the difficulty of passing the intervening envelope, and by the weakening of the action of the cloud by the increase of distance.

On the 19th April, 1827, the packet boat New York was twice struck by lightning. On the first occasion, having no conductor, it received considerable damage; on the second, the conductor was fixed; it was made of a pointed bar of iron, 1·2 metre (about 4 feet) long, and 11 millimetres (·43 inch) diameter at the base, and a surveyor’s chain about 40 metres (131 feet) long, forming a connection between the foot of the rod and the sea; the chain was made of iron wire 6 millimetres (·24 inch) in diameter; the links were 45 centimetres (17·7 inches) long, ending in loops, and joined together by two round rings. When struck the chain was dispersed in burning fragments and globules, which set the deck on fire in many places, notwithstanding the hail upon it and the rain which fell heavily; the bar at the top was melted for a length of 30 centimetres (11·8 inches) from the point, and down to a diameter of 6 millimetres (·2 inches). The rest of the rod remained with about 8 centimetres (3·1 inches) of the chain attached to it, the longest piece of chain found was less than 1 metre (3 feet 3 inches) long, and was blistered as by fire.

On the 13th June, 1854, the Jupiter was struck by lightning. The conductors were in place; that of the mainmast which was struck went 2 metres (6 feet 6 inches) into the sea, and had at its end a ball 2 kilos in weight. After being struck the conductor had disappeared and the pieces of it were scattered everywhere. The conductor, about 70 metres (230 feet) long, was a cable of three strands formed of sixty brass wires, each one half or two-thirds of a millimetre (·019 or ·026 inches) thick. The cable was mostly in bits no bigger than pins, but there were some pieces a few decimetres long, these had been turned violet colour as by fire, and those first touched were still burning hot.

These two examples show that a conductor may be destroyed, but they also show that it is not useless even then, since it will have received the discharge and directed it, and so prevented greater mischief. The Jupiter received no damage; whilst not far off, a Turkish vessel, which also had a conductor (but the chain of which did not reach the water) having been struck by lightning in the same storm, had a hole more than 30 centimetres (11·8 inches) deep, and almost such as would have been made by a cannon ball, in her side just above the copper, and near the water line.

The question is, are such accidents to conductors inevitable, or are they the result of faulty construction? All the facts established in the accounts of lightning and its phenomena, leave no doubt on this point. All the lightning rods which have been destroyed were of bad materials, insufficient, badly constructed, not in accordance with the principles which theory has deduced from experience.

The conductor of the New York had several faults; its upper terminal was too small, and too much drawn out; its conductor had much too small a sectional area; and the use of a chain in such cases should be strictly excluded.

There is no example known in which lightning has been able to melt iron rods 2 centimetres (·78 inch) in diameter, or 3 square centimetres (1·18 inch) in section; and copper may be used in still smaller sizes.

The conductor of the Jupiter, although better than the former, had also a radical defect. The fragments of the conductor which were examined bore but few traces of fusion, and none of these traces extended to the entire thickness of the cable; they were also limited to a group of some of the sixty wires of which it was composed. This seemed to show that the discharge was not carried equally by all the wires, and that those wires which it followed being insufficient to carry it, were the ones melted, and the others were broken or volatilised with explosion. Hence the breaking of the cable and dispersion of fragments of some decimetres in length, which, though too hot to be touched, were not hot enough to set wood on fire. This explanation, however, raises a singular question, whether, in a cable of similar wires twisted and bound together, the lightning can choose some wires in preference to the rest, even when the whole of them are hardly sufficient to give it a free passage.

Undoubtedly, yes; at any rate under certain conditions. No doubt if at both ends of the cable, for the length of a decimetre, the wires first tinned separately are afterwards soldered together, so as to make a sort of metallic cylinder, electricity, whether natural or artificial, having to pass along the cable, will not show a preference for one wire over another; but where this is not done—if at the two ends, or, more generally, at the two points of junction with other conductors, the wires are isolated by layers of dust or oxide—if, in addition, the cable only touches the terminals by its outside wires, then things happen very differently. The electricity takes those wires that are in contact with the terminal; these reduced to few in number become incapable of carrying it; and the whole cable broken by the explosion exhibits the phenomena shown in the case of the Jupiter.

The deficiency in each case was due to one cause—insufficiency of sectional area. In the first case the insufficiency is apparent, the iron wires 6 millimetres (·24 inch) thick were nine or ten times too small; in the second, the insufficiency is more hidden, it results from badly made junctions.

The two most fundamental rules for the construction of the rod and conductors are—1st. That they shall have a sufficient sectional area. 2nd. That they shall be continuous and without a break from the point of the upper terminal to the common receiver (the earth). But this continuity may in strictness be interpreted in two ways: it may be said that two pieces of metal in contact form a sufficiently continuous connection; and it may be said, on the other hand, that most frequently this simple contact is no more than a break in consequence of oxidation and the interposition of foreign bodies.

The instruction of 1823, without adopting the first interpretation, does not appear to have sufficiently recommended the second, which should exclusively regulate all construction of lightning rods. No doubt it is possible, by taking great care, to join and bolt together two pieces of iron or copper closely enough to make a practically continuous conductor, but when there are many joints we fear that evil might arise from the negligence of workmen, and still more from the chemical alteration of the surfaces, the deposition of foreign matter, and the mechanical dislocation produced by time and repeated shocks.

Hence, the three following practical rules should always be observed:—1. To reduce as much as possible the number of the joints. 2. To make all the joints with hard solder, and they should be upon surfaces of at least 10 centimetres (3·9 inches) square, and further strengthened by straps and bolts. 3. Not to make the upper terminal so gradually pointed as usual. The upper terminal of iron should be not less than 2 centimetres (·78 inch) diameter, the end should be filed down and a screw tapped 1 centimetre (·39 inch) high and 1 centimetre diameter, and to this a cone of platinum 2 centimetres diameter and 4 centimetres (1·5 inch) high, and consequently having an angle at the point of 28° or 30° should be fitted, screwed, and carefully soldered.

In other respects the instructions of 1823 should be followed; no fact which leads to a modification of the general rules there proposed 1, for the sectional area of the conductors; 2, for the method of fastening to buildings; 3, for the method of making the earth connection, has since come to light.

The subject, however, is not exhausted, there still remains the important and difficult question: what is the circle of protection afforded by a well constructed lightning rod? The opinion generally received at the end of the last century was that the circle of protection had a radius of twice the height of the terminal, and the instruction of 1823 adopted this opinion, but with some restrictions as in the case of spires. It is important to remember that these rules rest upon a more or less arbitrary basis, and this is said not to condemn them, but only to prevent there being attached to them a value which they do not possess.

More observations are required, and it is only with reserve that these rules are admitted. They are neither general nor absolute, they depend upon a variety of circumstances, and especially on the materials of the buildings. For example, the radius of the circle of protection, which would be sufficient for a building having only wood tiles or slate on its upper portion, would not be sufficient for a building in which the covering or the framing of the roof was of metal. In the former case the active portion of the thunder cloud, although further from the lightning rod than from the roof, would exert a greater action on the rod, whilst in the latter the action on the rod and on the roof would be almost equal at an equal distance.

A special note upon ships, and another on the Palais de l’Exposition close the report.

Special Report for the New Buildings of the Louvre, 18 December, 1854, by the same Committee.

Referring to the subject of the earth connection the Committee say; in the earliest instructions, it is said that the conductors should communicate with the water in a river, a pond, or wells, or at least with moist earth. This rule, although quite correct in itself, frequently leads to erroneous practice. It is sometimes thought that lightning is extinguished by water, as fire is; and when water is scarce the conductors are plunged into a well-cemented cistern. This is a most dangerous mistake; the conductor should be in connection with the common receiver, that is, the great water-bearing strata (nappes d’eau,) of much greater extent than the thunder cloud. At other times where wells are possible but costly, advantage is taken of the alternative allowed by the instructions. Instead of wells the conductors are put in connection with the earth, without being careful to see that it preserves sufficient moisture in times of drought when storms are most to be expected, and without being careful to see that the moist connection is sufficiently large. They specially note this latter error, as it appears to be still more common than the former. They do not hesitate to say that recourse should never be had to this method of connection with the common receiver. They recommend that in default of rivers or very large ponds, the conductor should always be connected by large surfaces with the inexhaustible subterranean water-bearing strata.

Secondly, where these strata are at a moderate depth below the surface, the Committee consider it necessary to make use of a conductor with two branches, the principal to descend to the subterranean water; the secondary, leaving it at the ground level, is put in connection with the surface. And for this reason; after great droughts thunder clouds exert but a feeble influence upon a dry, badly conducting soil. All their energy is felt by the subterranean waters; and the electricity will be carried by the principal branch. On the other hand, after a summer shower, when the surface soil gets moist, it is at once made a good conductor. It is that which is affected by the thunder cloud: while, at the same time, it screens the subterranean water from electrical influence. In such a case it is indispensable that the surface of the ground should be in direct connection with the conductor; and this the secondary branch supplies.

There is a final question how the conductors should be connected with the various metallic portions of the building. The ridges are throughout of iron; but the interior arrangements require that, in some portions of the building, there should be, properly speaking, only one floor, whilst in other parts there are six. Each floor may be regarded as a great metallic network, composed of several strong plate girders, crossed by numerous joists analagous to rails, while these are, in their turn, crossed by a multitude of smaller iron rods; and the meshes of this network are filled with tiles. In enquiring into the effect of a thunder storm upon those portions where there are six such floors one above the other, it is easy to see that if the roof were a great continuous sheet of metal, it would take up the whole electrical energy of the cloud, at any rate, as far as the floors underneath it are concerned. In this case it would be amply sufficient if the covering were well connected with the lightning rods. But in this case the roof is metallic, only in a very small portion; it may be said that the ridges only form a network with very large meshes, and, consequently, is an insufficient shield, through which the upper floor may still receive a considerable shock. Therefore the Committee propose the following arrangements:—1st. The principal pieces of each floor should be put in connection with the conductor. 2nd. It is very desirable that all the joists of the upper floors should be connected together by a rod bolted, and, if possible, soldered to each, which rod should be connected with the conductors. 3rd. It seems probable that, in general, the roof frames are in good connection with each other, and, consequently, it would suffice if all the upper terminals are connected with them. If, however, it happens either by changes of level in the gutters, or from other causes that the connections become doubtful special iron connections must be made. 4th. The zinc gutters and ridges should be connected with the lightning rods.

The committee examined the points presented by Messrs. Delieul, one of platinum, made exactly as described in the report of the previous 18th December; the other, a cone similar in form, size, and external appearance, but rather less costly, being made of a cap of platinum, fixed with hard solder upon the conical end of the iron rod. It was thought that this second arrangement would not practically be inferior to the other; but it must be made by a skilful workman, who knows how to insure that the solder should take to the whole of the surfaces brought together. They see no objection to the substitution of palladium, or gold or silver of a standard of ·950 for the platinum. But all these metals are costly; few workmen know how to work in them, or at least to employ that precision, and take that minute care, which are indispensable to success. These reasons have raised again a proposition that was discussed in the former commission, which consists in making the points of copper. The copper point is 2 centimetres (·78 inch) in diameter, like the upper part of the iron rod, to which it is screwed and brazed; its length is about 20 centimetres (7·87 inches), and it terminates in a cone 3 or 4 centimetres (1·1 or 1·5 inches) high.

They see no reason why this should not be used with almost the same confidence as the preceding forms. If there is ground to fear that it may undergo changes from atmospheric influences, this is counterbalanced by certain advantages. 1st, copper is with palladium, gold, and silver, among the best conductors of heat and electricity; and the point of the cone will be much less heated than the platinum point; and 2nd, the terminal, with a copper point, is much less expensive, and can be made everywhere.

On the report being put to the vote M. Despretz could not approve the proposal to employ copper points, fearing that the deposition of carbonate or some other badly conducting matter would diminish the efficacy of the lightning rod.

INSTRUCTIONS upon LIGHTNING RODS for POWDER MAGAZINES, by a Committee consisting of MM. Becquerel, Babinet, Duhamel, Fizeau, Edm Becquerel, Regnault, le MarÉchal Vaillant, and Pouillet.

After referring to some general principles, and to the construction of lightning rods recommended in the reports of the earlier Committees: the Committee recommend, that the upper terminal including the copper point should be from 3 to 5 metres (9 feet 10 inches to 16 feet 5 inches) high; that the junction of the conductor and the upper terminal, and also the several joints of the conductor, should be covered with solder, and insist very strongly upon the necessity of communication with the nappe d’eau souterraine, which they define as “the water level in neighbouring wells which never dry up, and which retain at least 50 centimetres (19·68 inches) in depth of water in the most unfavourable seasons.”

The special arrangements to be adopted in setting up lightning rods for powder magazines are: not to fix them on the building itself but outside the surrounding walls. For each large sized magazine (27·89 metres, by 20 metres, and 11 metres high, equal to 91 feet 6 inches by 65 feet 7 inches, and 36 feet high) there should be three conductors—two near the ends of the long side of the enclosing wall most exposed to storms, and the third in the middle of the opposite side. The upper terminals should be only 5 metres (16 feet 5 inches) high, and should be raised on a pier, a mast, or other support 15 metres (49 feet 2 inches) high, down which the conductor should be led to the ground. There should be a circuit which the Committee call circuit de ceinture carried entirely round the enclosing wall to which each conductor should be joined, and a conductor should be carried from the most convenient point of this circuit to the underground water. For middle sized magazines two terminals and supports, and for small magazines one terminal and support will suffice; but in all cases there should be a circuit de ceinture. This need not be deep below the surface, nor covered over; it may even be in an open gutter, but a conductor must be led from it to the underground water, even if in order to do this it is necessary to carry the conductor several hundred metres or several kilometres. It need not, however, be made of bars and carried all the way in a trench, but it may be made of six wires 6 or 7 millimetres (about ·25 inches) in diameter, and carried on posts like telegraph wires, except that they need not be insulated.

INSTRUCTIONS by the Committee consisting of MM. Alphand, Belgrand, Fizeau, Comte du Moncel, Ed. Becquerel, Desains, Ch. Sainte Claire-Deville, Duc, Ballu, Magne, Davioud, Felix Lucas, and R. Francisque Michel, appointed to inspect the Lightning Rods on the Municipal Buildings of Paris.

The Committee find that platinum tips are useless, and recommend instead that the point of the terminal should be made of pure copper, 50 centimetres (19·7 inches) long, and terminating in a cone, forming an angle of 30°. This should be scarfed, pinned, and soldered to the end of the terminal. The terminal should be of wrought iron in one length, and where possible galvanized; but on no account painted. The connection with the conductor should be by a piece fitted and bolted; and, lastly, the whole joint should be well covered with solder.

The Committee consider that on an ordinary building a terminal will effectively protect a cone, having the point for its apex, and a base whose radius is 1·75 of its height. But in practice the terminals may be much farther apart, if there is a circuit des faites. This is defined as a metallic conductor, which extends without break over the ridges of all the buildings which it is intended to protect, and which is joined by metallic contact to all the upper terminals and to the conductor, and consequently to the underground water which alone forms the common reservoir. All pieces of metal of any considerable size should be connected with the conductor.

If the conductor is made of iron bars, they should be galvanized if possible, and the joints should be fitted, bolted, and finally covered with solder. If the bars cannot be galvanized, they should be well painted. The Committee recommend the employment, especially in the circuit des faites, of an arrangement for compensating for the lengthening and shortening of the bars by the variations of temperature. This is made by inserting in the circuit a curved band of copper which will yield to the movement of the rods. If the conductor is made of galvanized iron wire rope, each wire should be 2·5 or 3 millimetres (·09 to ·11 inch) in diameter, and there should be such a number of them that the sum of their sectional areas shall be equal to one-fifth more than that of a bar of iron 20 millimetres (·78 inch) square. The rope should be all in one piece, and the joints with the terminal and earth connection should be covered with solder.

The supports should not be insulated, and there should be as few as possible of them. At the underground end of the conductor should be fixed a large sheet or hollow cylinder of metal, and this should be always, even in the greatest droughts, plunged at least 1 metre (3 feet 3 inches) into the subterranean water. If from any cause this water cannot be reached, the conductor may be joined to one of the main water-pipes of the city; but if the conductor cannot be led either to the subterranean water or to a main water-pipe, no lightning-rod should be erected. It would do more harm than good.

In the case of buildings of any importance, two or more conductors leading to the subterranean water should be employed. It should be so arranged that the underground part and the earth connection may be easily inspected and cleaned from rust, and the whole should be inspected and cleaned at least once a year, at the end of the autumn. The Committee is of opinion that it would be better to put all the lightning-rod work in the hands of special workmen, under the control of an agent appointed by the administration, and not to trust it to the blacksmiths and locksmiths usually employed. The Committee lastly recommend that they should be permanently appointed, and meet every year after the inspection, to report and decide upon the steps to be taken to remedy any defects that may be discovered.

REPORT by the joint Secretary (Francisque Michel) of the Lightning Rod Committee to the Prefect of the Department of the Seine.

This report gives a detailed description of the state of the lightning rods attached to the public buildings of Paris.

In most cases the upper terminals were of great length, some of them as much as 9 metres (nearly 30 feet) in height; the conductors were in almost all cases of iron, either in bars or wire rope; the earth connections were of various kinds and extent.

The report frequently states that the points were blunted; that the upper terminals and the conducting rods were deeply rusted; that especially at the joints the conductors were seriously deficient; and that the underground portion was greatly deteriorated by rust.

A description is given of an accident from lightning to the church of St. Sulpice; but this building had no lightning rod. In the case of the church of St. Clotilde there are five upper terminals, two on the two spires, the remaining three along the ridge of the main roof. The building was amply protected as far as its length was concerned, but the transept was not so thoroughly protected. The five terminals were joined to a conductor which went round the building, and was connected with the ground. A second conductor led from one of the terminals to the ground, where it terminated in a second pit. The conductors were made of iron rods 18 millimetres (·71 inch) in diameter, joined by collars and pinned and the whole covered with paint. They terminated in distributors plunged in the underground water in walled pits. They were supported by insulated collars. The building has an iron roof. The church had been struck by lightning at least four times since the lightning rods had been erected. The first time, twelve years ago, the lightning struck the rod placed on the transept, and carried away the platinum tip of the copper point. Since then the rod has received another discharge, and the copper point is bent to the S.W. In January, 1872 or 1873, the lightning struck the western tower, and shattered one of the stones above one of the windows of the staircase.

“One of the platinum tips is gone, and many are blunted. The conductivity of the conductor is very bad, and the joints are very much damaged: hence the accident to the tower. The greater number of the glass insulators are broken, or gone altogether.”

In the case of the church of St. Eloi, which had one terminal on the spire, one conductor, formed of iron wire rope 2 centimetres (·78 inch) in diameter, joined at 3 metres (9 feet 10 inches) above the ground to an iron rod 25 millimetres (·97 inch) in diameter, which entered the ground and ended without branches in a pit filled with charcoal. The soil was dry and calcareous. The conductor was made up of many lengths of rope, old pieces apparently having been used; the joints were in bad condition, and needed soldering. The underground part was deeply rusted.

“In September, 1874, lightning struck the spire, twisted the conductor, broke the terminal, threw down the part above the cross, and made great cracks in the apse.”

During the building of the Mairie of the 20th Arrondissement, the lightning struck a fir-pole in the scaffolding. It did not do any damage, being carried away by the chain attached to the pole, from which it took all the rust, and being thence conducted by some pieces of iron roof framing lying on the ground.

There are several other accounts of accidents, but they are mostly represented by the foregoing examples.

1. The principles adopted by Sir W. S. Harris, as shown in the Appendices A and B, to this paper, still held to be sound.

2. The terminating plane of action of lightning is sometimes beneath the surface of earth, which, if moist, forms good medium for diffusion of electricity.

3. Dry soil is to be regarded as non-conducting matter.

4. Therefore conductor to be taken into soil permanently damp.

5–6. Underground magazines are usually in dry soil, and should

therefore be fitted with conductors as in the case of similar magazines above ground.

8–9. Casemated batteries of modern construction, with magazines in basement should have conductors on the parapet or terreplein from end to end of battery, attached to vertical conductor into earth. Flagstaff should have conductor. In large works there should be several points 5 feet above top of building. Iron verandahs and railings are good conductors when with good earth connections.

10. Iron buildings are good conductors. But if covered with asphalte, concrete, &c., rods or points must be provided projecting above asphalte, &c., and with good earth connections. Iron shields should be connected with conductors.

11. Copper is recommended as best conductor; it is not liable to corrosion, and very durable.

12. But if exposed to injury, or likely to be stolen or corroded, copper may be replaced by iron, provision being made for its smaller conductivity—viz., ?th that of copper.

13. Copper rods to be ½ inch diameter; copper tubes to be ? × ? inch thick; copper bands to be 1½ × ? inch thick.

14. If the conductor be of iron, solid rods to be 1 inch diameter; solid bands to be 2 inches wide × ? inch.

15–16. The fusing temperature of copper is 1994° Faht.; whereas that of iron is 2786° Faht. So far there is a marked advantage over copper. But it rusts easily, and then the electrical resistance is immensely increased. Roughly speaking, an equal conducting power may be obtained either in iron or copper for the same cost, the number of iron conductors being greater in proportion to the less cost, and the more conductors being the better.

17–19. Expansion and contraction are to be carefully provided against; e.g. by suitable bends at intervals in long lines of horizontal conductors and by bearing collars, allowing of slip in vertical lines.

20. Soldered or welded joints are desirable, but not absolutely necessary.

21. Gives engravings of connections recommended by Sir W. S. Harris, where soldered joints cannot be used, and which fulfil the conditions specified in sections 17–19.

22. Soldered or welded joints to be used where discharge is possible with unsoldered joints, and likely to ignite dust or inflammable substances near.

23. Iron may be connected by similar joints as for copper, or by screw joints as for gas pipes. No white lead to be used, it being a bad conductor.

24. Iron flat bands may be connected by rivets or screws, working in slots, to provide for expansion, each surface in contact being at least six times the sectional area of band.

25. Copper bands to be similarly connected. Joints between different metals may be soldered, screwed, or rivetted, the extent of surface in contact being regulated by the dimensions of the metal of the least conducting power. Access of moisture to surfaces in contact must be prevented, on account of local galvanic action and decomposition.

26. No precise limit can be fixed to protecting power of conductors. In England the limit is usually assumed as being the radius of the height from ground. It may be sufficiently correct for practical purposes, but cannot always be relied upon.

27. Conductors do not attract lightning; they only diminish the resistance due to the air. Even a change in the nature of the soil -over which a cloud passes may produce a discharge.

28. One angle of a building may receive a discharge, though another angle have a conductor. So every prominent part of a building containing explosive material should have a conductor.

29. In buildings of uniform height, provide a solid rod 5 feet above it at each end, and at each 45 feet in length; if the conductor be of iron the top should be gilt.

30. Buildings not over 20 feet long to have one vertical conductor at end, and a horizontal conductor on ridge.

31. If 20 to 40 feet long to have one vertical conductor in centre, and one along ridge, as last.

32. If 40 feet long to have two vertical conductors; if 100 feet long three conductors; in both cases with conductor along ridge.

33. Similar principles to be adopted in larger and more complicated buildings.

34. Each prominent part should have a conductor. The value of three or four points to terminals is not apparent unless the points are widely separated.

35. Conductors are to be connected horizontally, e.g., by ridge or eaves, which, when of metal, should be invariably connected with conductor. All metal surfaces whatever to be also so connected.

36. Sir W. S. Harris considers the relative conductivity of the several metals as being—of lead 1, tin 2, iron 2½, zinc 4, copper 12. So lead cannot be altogether depended on.

37. Avoid long lengths of horizontal conductors without earth contacts, as the currents might leave the conductor, and pass to earth, causing danger. Avoid sharp angles.

38. Good earth connections most important. Conductors are to be led into springs or wells or earth permanently wet. Not into watertight tanks. Shingle, dry sand, or dry mould are not sufficient. Provide several earth connections in all large systems of conductors as a precaution.

39. Lead conductors into ground in trenches 18 inches deep. Not less than 30 feet of metal to be in contact with moist earth.

40. Lead a flow of water over trenches if possible, e.g., from rain water pipes.

41. Trenches in rocky or dry soil to be 30 to 120 feet long, so as to obtain all moisture possible.

42. Connections in trenches may be of old iron, forming continuous metallic surface, the trenches to be filled with cinders or coal ashes. Water pipes form excellent earth connections, but gas pipes are dangerous.

43–44. Frequently inspect conductors, especially as to joints connecting different metals and defects in iron from rust.

45–46. Galvanize iron, care being taken that the coating is good.

47. Great care to be taken in case of contact of zinc coating with other metals, especially copper.

Appendix A.—By the late Sir W. Snow Harris, F.R.S.

1. The earth’s surface and clouds are the terminating surfaces of electric actions, and buildings, &c., are only points, as it were, of earth’s surface in which the whole action vanishes.

2. Electricity when confined to substances resisting its progress, as air, glass, dry wood, stones, &c., exerts a terribly explosive power.

3. But when confined to bodies, such as metals, offering small resistance, its violent expansion or disruptive action is greatly reduced or avoided altogether, and becomes a continuous current comparatively quiescent. But if body be small, as wire, it may be heated or fused. Resistance is so small that a shock has traversed copper wire at the rate of 576,000 miles a second; resistance increases with length and diminishes with area of section of conductor.

4. So a building metallic in all its parts, or a man in armour is safe.

5. So endeavour to bring buildings into the same passive or non-resisting state as if of metal.

6. So conducting channels of copper should be systematically applied to walls, either in plates united in series one over another, not less than 3½ inches wide and 1/16th and ?th of an inch thick, or of stout copper pipe not less than 3/16ths of an inch thick, and 1½ to 2 inches diameter, fixed to building by braces or copper nails or clamps. Terminals to be solid metal rods, projecting above to a moderate and convenient height. Earth connections to be by one or two branches, leading out about a foot below ground—if possible into moist ground, but if dry, use old iron or other metallic chains so as to expose a large metallic surface.

7. All metals in roof, &c., of building to be connected with main conductors; any prominent chimney to have a pointed conductor taken along it to metals of roof.

8. An electrical discharge never leaves a perfect conductor to pass to a very bad one, so the apprehension of lateral discharge is absurd. Furious discharges have fallen on the conductors to the masts of H.M. ships, and passed through copper bolts in bottom without injury even to persons leaning against the conductors.

9. Metallic bodies have no specific attraction for electricity more than wood or stone have; all matter is indifferent so far as regards a specific attraction. Lightning falls indiscriminately upon trees, rocks and buildings, whether with metals about them or not; e.g., at Plymouth Dockyard in May, 1841, a granite chimney, 120 feet high, without any metal in it, was struck, and yet it was within 300 feet of a clock-tower of equal height, having metal weathercock, a dome covered with metal and large conductor along it to ground. The damage ceased where the chimney passed through a massive metallic roof, having a conductor from it to the ground. Here the lightning fell on a building, which, according to the popular idea, held no “invitation” in preference to a structure which did hold such “invitation.”

10. If efficient conductors provide free and uninterrupted course for electrical discharge, it will follow that course without danger to general structure; if not, then this irresistible agency will find a course for itself and shake all imperfect conducting matter in pieces in doing so. The great object is to provide a line or lines of small resistance in given directions, less than the resistance in any other line of the building. The conductor no more attracts lightning than a gutter or water pipe attracts a flow of water.

11. It follows that a magazine if of metal would be safer than if built in the usual way. Metallic gutters and ridges, with continuous metallic communications to earth, are unobjectionable.

Note.—It is as wrong to isolate conductors from buildings by glass or resin, as it would be to place rain water pipes 10 feet from the building from which they should carry off the water.

An instance is given of an iron conductor which was placed 10 feet from a house, the latter being, notwithstanding, struck at the point nearest to the conductor, which was untouched.

12. Pointed terminations tend to break the force of lightning when it falls on them. Before explosion a large amount of discharge passes off through pointed conductors.

Pointed conductors should be solid copper rods, about ¾ inch diameter, and a foot long, united by brazing to the conducting tube. It is not necessary to gild the points, or form them of platinum. Sometimes even, this would be detrimental, as platinum has only half the conducting power of copper. The oxidations of the surface of conductor is of little moment; and in case of copper very trifling. In any case the conducting surface is better than the bad—or non-conducting air. The electric telegraph wires work well, though enclosed by gutta percha or other non-conducting matter. It is sufficient if the terminal solid rod be even roughly pointed. But even a ball, a foot diameter, would be a point as opposed to 1,000 acres of charged clouds.

Note.—Experience contradicts the idea that the conductor protects a certain area. The foremast of a ship has been struck, though the mainmast has been protected by conductor.

13. Copper linings to doors and windows of magazines, are not objectionable, but useless for keeping out lightning. They should be connected with the general system of conductors.

Appendix B.—As to Solid or Hollow Conductors. By Sir W. Snow Harris, F.R.S.

1. A given quantity of electricity melts the same quantity of metal, whether in a solid or hollow form. So far it is immaterial which form the conductor has. But supposing the mass of metal to be so large that the heating effect may be neglected. It is proved that the greater the surface, the less is its intensity or power at any point, the intensity approaching the second power or square of the surface inversely. It is important to give the charge free room of expansion by increasing the surface of conductor, so as to reduce the mechanical activity of shock to the least possible. Rectangular flat bars may be employed.

2. A rain water pipe communicating with main conductor, should have earth connection. All imperfect substances, as masonry, and ship masts, transmit a certain portion of electricity without explosive action. One great use of the conductor is to relieve the wood or masonry of the quantity it cannot discharge without explosion.

3. Conductors of small iron rod or wire are very objectionable. They commonly rust at the joints, and have fallen to pieces, and often been knocked to pieces by lightning. Iron may, certainly, be employed with advantage, but should be galvanized. Zinc is an even better conductor than iron; and being spread over the surface is not open to the objection of making a conductor of two metals of unequal conducting power. A good and efficient conductor might be formed of galvanized iron. It should be of wrought iron, galvanized, of 2 inches diameter, with screwed joints of extra thickness. Copper tubing is, however, always to be preferred.

4. In dry or rocky soil, complete the conductor by leading old iron chains out from the walls in several directions, or by leading a flow of water over them. Fortunately a thunder storm is usually attended by heavy rains. The iron chains should extend 30 to 50 feet, and be a foot or 18 inches under ground. The termination in a large surface of moist earth would be preferable to that in a well, as the action is a superficial one of expansion in all directions. In the tin leaf coatings of the electrical jar, the charge is not influenced by the thickness of metal.

REPORT ON THE DESTRUCTION BY LIGHTNING OF A GUNPOWDER STORE AT BRUNTCLIFFE, YORKSHIRE. By Major Y. D. Majendie, R.A.

The Gunpowder exploded at 4.30 p.m. on August 6th, 1878, during the greatest intensity of a violent thunderstorm. The building, was brick, with brick arched roof, length 9 feet, width 5 feet, height 6 feet (internal dimensions). The store had a uniform thickness of three bricks, and was furnished at the one end with an iron door, at the other end with a lightning conductor. The conductor consisted of a copper wire rope, 10 gauge copper wire, the rope being 7/16 inch thick, having four points at the top (one large one in the centre, and three smaller ones round it), it extended to about 13 feet above the top of the building, and about the same length was carried into the ground and terminated in a drain. The conductor had been erected in 1876, by Mr. John Bisby, of Leeds, and was fixed to a pole distant about 2 inches from the end of the building opposite to that in which the iron door was fixed, it was not connected with the iron door in any way. No one was near the store when the powder exploded, and it seems probable that the earth connection of the conductor was bad, that the mass of iron in the door offered at least an equally good path—and that the gunpowder was ignited by a flash passing between, the two imperfect conductors.

“The only structural damage effected was produced by the impingement of bricks, which striking with great force, had in a few instances, partially penetrated or displaced brick work in the dwelling-houses and buildings, and a portion of the iron of an iron church was broken by a piece of projected dÉbris. A brick was driven through a window in one of the houses at three hundred yards, and broke a bedstead. As far as I have been able to discover no other structural injury was occasioned.”

This accident appears to suggest several conclusions:—

“In the first place it appears to me to afford a striking confirmation of the principle which has been repeatedly and emphatically enunciated by Sir William Snow Harris and other authorities on the subject of lightning conductors, that in order to secure an efficient protection for a given building, all the metal of the building, and as far as possible the whole of the structure itself, should be brought into actual connection with the system of conduction; in other words, that the general conducting power of the mass of the edifice should be completed, and all attractive and prominent parts allied in one protective combination, so as to “bring the whole” (as it has been expressed by Sir William Snow Harris) “as nearly as may be into that passive or non-resisting state which it would assume, supposing the whole were a mass of metal.” In the present case, assuming the conductor itself to have been efficient, a point which there seems no sufficient reason for doubting, the system of conduction was obviously defective. Not only was the whole length of the building left unprotected, the conductor having been on a pole at one end, and carefully insulated from the building, but the iron door which was at the opposite end, was absolutely unconnected therewith, and was not itself supplied with any earth connection.”

“It appears clear, therefore, that even what may be deemed per se an efficient lightning conductor, i.e. a conductor, which considered alone, offers a path of little or no resistance even to a powerful electric current, does not afford a reliable protection to a building unless it be scientifically applied, and with due regard to those principles upon which the more eminent authorities on electrical science are agreed. To a disregard of these principles, especially in respect of the iron door being left out of the system of conduction, and unconnected therewith, I believe the present accident may be attributed.”

REPORTS OF COMMITTEES ON THE POWDER MAGAZINES AT PURFLEET.

Report of a Committee consisting of the Hon. Henry Cavendish, Dr. Watson, Dr. Franklin, Mr. J. Robertson, Mr. Wilson, and Mr. Delaval, appointed by the Royal Society, “to consider of a method for securing the powder magazine at Purfleet.”

A powder mill at Brescia having blown up in consequence of being struck by lightning, the Board of Ordnance applied to Mr. B. Wilson to know in what way the powder magazine could be protected. He recommended that a blunt conductor should be employed, whereas Dr. Franklin recommended a pointed conductor. The Committee met and Dr. Franklin read a paper on the subject, and the report of the Committee was in conformity with Dr. Franklin’s views.

The Committee went to Purfleet and examined the buildings. They found that the barrels of powder, when the magazines were full, lay piled on each other up to the spring of the arches; on each barrel were four copper hoops, which with vertical iron bars formed broken conductors within the building. These iron bars were ordered to be removed.

The Committee advised that at each end of each magazine a well should be dug in or through the chalk, so deep as to have in it at least four feet of standing water. From the bottom of this water should arise a piece of leaden pipe to or near to the surface of the ground, where it should be strongly joined to the end of an upright iron bar, an inch and a half in diameter, fastened to the wall by leaden straps, and extending ten feet above the ridge of the building, tapering from the ridge upwards to a sharp point, the upper twelve inches of copper, the iron to be painted.

Lead was mentioned for the underground part as less liable to rust, in the form of a pipe as giving greater stiffness for the substance, and iron for the part above ground as stronger, and less likely to be cut away. The pieces of which the bar may be composed should be screwed strongly into each other by a close joint with a thin plate of lead between the shoulders. Each rod in passing above the ridge should be strongly and closely connected by iron or lead, or both, with the leaden coping of the roof, so making metallic communication between the two bars of each building.

It was also advised that two wells be dug within twelve feet of the doors, one to the north of the north building and the other to the south of the south building, and that metallic communications be made between the water in them and the leaden coping of the roof.

The Board house stood 150 yards from the magazines, on elevated ground, and was a “lofty building with a pointed hip-roof, the copings of lead down to the gutters, from which leaden pipes descend at each end of the building into the water of wells of forty feet deep, for the purpose of conveying water forced up by engines to a cistern in the roof.”

As to the Board-house, they thought it already well furnished with conductors by the several leaden communications above-mentioned from the point of the roof down into the water, and that by its height and proximity it may be some security to the building below it; they therefore proposed no other conductor for that building, and only advised erecting a pointed iron rod on the summit, similar to those before described and communicating with those conductors.

Mr. Wilson dissented from that part of the Report which recommended that each conductor should be pointed, because, he says, “by points we solicit the lightning, and may promote the mischief by drawing the charges from charged clouds, which would not discharge at all on the building if there were no points on the conductors.” By experiments made and appealed to at the Committee the difference in the effects between pointed and blunted conductors is as twelve to one. Mr. Wilson states that, “A thunder cloud, therefore, if it acted at 1200 yards distance upon a point, would require a blunted end to be brought within the distance of 100 yards, and beyond those limits would pass over it without affecting it at all.” He also says, “The longer the conductors are above the building, the more danger is to be apprehended from them. I have always considered pointed conductors as being unsafe by their great readiness to collect the lightning in too powerful a manner.”

Mr. Wilson adds an account of an accident to St. Paul’s Church, and some curious reasoning on it in support of his own views. (See Phil. Trans. 1773, p. 59–61.)

---

On the 15th of May, 1777, the Board House at Purfleet was struck by lightning, and some of the brickwork damaged (See Phil. Tran., 1778, Pt. I., p. 232). About 6 p.m., after heavy rain through the day, a heavy cloud hung over the house for some time, and Mr. Nickson, who watched it from the house and gives the account, says he suspected that some of the conductors might find employment from it. He had not been long at the window before a violent flash of lightning and clap of thunder came together. The lightning struck one of the iron cramps that hold the coping, and made a dent in the lead of the cramp and the stone adjoining it, throwing some stone down and slightly disturbing about a cubic foot of brickwork at A. The iron cramp was situated over a plate of lead, and the ends of it, inserted in the stone, came within 7 inches of that plate, which communicated with the gutter, and served as a fillet to it; this gutter was part of the main conductor of the building. The lightning struck through the stone, &c., to the corner of the plate, fusing a very small portion of it. From this point no farther effect of the lightning could be traced. At the distance of seven feet and a-half from the place struck, a large leaden pipe went down from the gutter to a cistern of water in the yard. It is remarkable that the surface of one of the hip-rafters, four inches and a-half in diameter, covered with lead (communicating with the gutter), and reaching within twenty-eight inches of the place struck, seems not to have been at all affected. The distance from the point of the conductor on the house to the part struck was forty-six feet.

A fresh Committee of the Royal Society, consisting of Mr. Henly, Mr. Lane, Mr. Nairne, and Mr. Planta, recommended a channel to be made from cramp to cramp round the parapet, filled with lead, and connected in four places with the main conductor on the roof of the building.

Mr. Wilson again dissented from their report, and attributed the hanging of a heavy cloud over the house (it being calm at the time) to the presence of the pointed lightning conductor.

An account of Mr. Wilson’s elaborate series of experiments at the Pantheon on a long cylinder to illustrate the effects of pointed and rounded conductors occupies seventy pages of the Philosophical Transactions; and another Committee of the Royal Society, consisting of Sir John Pringle, Dr. Watson, Henry Cavendish, W. Henly, Bishop Horsley, T. Lane, Lord Mahon, E. Nairne, and Dr. Priestley, report in favour of having additional conductors ten feet high, with copper eighteen inches long, finely tapered and acutely pointed placed upon the magazines. They conclude that “elevated rods are preferable to low conductors terminated in rounded ends, knobs, or balls of metal,” conceiving that, the experiments and reasons, made and alleged to the contrary by Mr. Wilson, are inconclusive.

Mr. Wilson’s objections are again urged by Dr. Musgrave, but called in question by Mr. Nairne (see Phil. Trans., 1778, Pt. 2, p. 823), who makes a series of experiments to illustrate the advantage of pointed conductors.

Both Mr. Wilson’s and Mr. Nairne’s experiments agree in showing that “pointed conductors draw off the electricity from a cloud at a much greater distance than those which are blunted.” Mr. Wilson objecting that this draws the charged cloud from a greater distance; and Mr. Nairne concluding that “a charged body is exhausted of more of the fluid by a pointed than by a blunted conductor,” and so is not likely to cause so much damage since it discharges itself more gradually.

EXPERIMENTS AND OBSERVATIONS ON ELECTRICITY.

The author shows that pointed bodies draw off electricity much more effectually than blunt ones.

When the land is hot, “the lower air is rarified and rises; the cooler, denser air above, descends.”

The clouds meet over the heated place, “and if some are electrified and others not, lightning and thunder succeed, and showers fall.”

“As electrified clouds pass over a country, high hills and trees, towers, spires, masts, chimneys, &c., as so many points, draw the electrical fire and the whole cloud discharges there.”

Therefore it is dangerous to take shelter under a tree. It is safer to be in the open fields, especially if the clothes are wet.

Metals are fused, possibly without heat; the lightning creating a violent repulsion of the particles of the metal it passes through.

[He afterwards admits this opinion to be erroneous.]

Describes experiments with sharp-pointed metallic bodies, and says: “May not the knowledge of this power of points be of use to mankind in preserving houses, churches, ships, &c., from the stroke of lightning, by fixing on their highest parts upright rods of iron, made sharp as a needle, and gilt, to prevent rusting; and from the feet of these rods lead iron wire down the outside of the building into the ground; or down one of the shrouds of a ship and her side till it reaches the water.”

“Would not pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?”

He mentions the case of the topmast heads of a ship being struck, but having flames upon them like very large torches before the stroke.

He thinks that if there had been a good wire conductor from the heads to the sea there would have been no stroke or damage.

He records the experiments on the 10th of May, 1752, at Marly, of M. D’Alibard, who placed upon an electrical body a pointed bar of iron 40 feet high. In a thunder storm sparks of fire were attracted from it.

Again, at Paris, on the 18th of May, with the same result, by M. de Lor, with a bar of iron 99 feet high upon a cake of resin 3 inches thick and 2 feet square.

Similarly in London in July, 1752, by Mr. Canton.

He refers to other experiments.

He experimented in 1752 with a kite of thin silk (as being able to bear the wet), having a very sharp-pointed wire fixed to its top, above which it rose about a foot. The kite was raised by twine, the part in the hand being made of silk and kept quite dry.

The pointed wire will draw the electric fire from thunder clouds, and when the rain has wet (sic) the kite and twine, so that it conducts the electric fire freely, they will be electrified, and the electric fire will stream out plentifully on the approach of the knuckle.

“Spirits may be kindled, &c., as with a rubbed glass or tube, and thereby the sameness of the electric matter with that of the lightning be completely demonstrated.”

September, 1752. He erected “an iron rod to draw the lightning into his house in order to experiment on it.”

After many experiments, he concluded that “the clouds of a thunderstorm are most commonly in a negative state of electricity, but sometimes in a positive state.” The latter, he believed, rare.

“So that, for the most part, in thunderstrokes, it is the earth that strikes into the clouds, and not the clouds into the earth.”

In the contrary (rare) case the cloud was, “I conjecture, compressed by the driving winds or some other means, so that part of what it had absorbed was forced out, and formed an electric atmosphere round it in its denser state, so communicated positive electricity to my rod.”

“The electric fluid, moving to restore the equilibrium between the cloud and the earth, takes, in its way, all the conductors it can find (v. page 132 of Franklin’s book)—as metals, damp walls, moist wood, &c.—and will go considerably out of a direct course for the sake of the assistance of a good conductor.”

“Explosions only happen when the conductors cannot discharge it as fast as they receive it, by reason of their being incomplete, disunited, or too small, or not of the best materials for conducting.”

He supposes that a wire ¼ inch diameter will conduct the electricity of any one stroke of lightning ever known.

Iron is the best material, as least liable to fuse.

“Pointed rods erected on buildings and communicating with moist earth would either prevent a stroke, or, if not prevented, would conduct it so that the building should suffer no damage.

He gives instances of a small wire acting as conductor and saving the building, though the wire, being too small, was utterly destroyed.

His theory as to the crooked course of lightning is as follows:

“Who knows but that there may be, as the ancients thought, a region of this fire (electric) above our atmosphere, prevented by our air and its own too great distance of attraction from joining our earth. Yet some of it be low enough to attach itself to our highest clouds,” which thence become electrified, &c.

“I am still at a loss about the manner in which clouds become charged with electricity, no hypothesis I have yet formed perfectly satisfying me.”

He describes how he and others have been struck down by electric shocks without feeling pain or sustaining permanent injury.

For protecting powder magazines, erect a mast not far from it, and 15 or 20 feet above the top of it, with a thick iron rod fastened to it, reaching down till it comes to water.

“In buildings the rod may be fastened to the walls, chimneys, &c., with staples of iron. The lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor) through these staples. It would rather, if anywhere in the wall, pass out of it into the rod to get more readily into the earth.”

If the building be very extensive, two or more rods may be placed at different parts for greater security.

It is well not to sit near the chimney, or gilt objects, during a thunderstorm.

AN ESSAY on the cause of LIGHTNING, and the manner by which the thunder clouds become possessed of their electricity, deduced from known facts and properties of that matter, to which are added plain directions for constructing and erecting safe conductors. By John Simmons. 8vo. 1775.

“As on the earth the operation necessary for the excitation and collection of the electric fluid is attrition.” ... “So we may rationally conclude that attrition is the means of excitation and collection of electric matter in the clouds as well as on the earth.”

By metallic conductors buildings may be preserved from the effects of lightning

Electricity ascends from the earth to the clouds by means of moist air.

“A conductor is a continuation of metal from a certain height above the highest part of a building to moist earth or water” ... “for easy and safe passage of lightning.”

Metal is the best of all conductors.

The author quotes from Franklin “buildings that have their roofs covered with lead and spouts of lead continued from roof into ground to carry off the water, are never hurt by lightning when it falls on such a building.”

The conductor may be made of any metal, and flat or round.

But nowhere less than ¾ inch diameter except at terminal.

But iron rusts, so copper or lead should be used. Lead is best, used in strips 4 inches wide and ?th inch thick.

Good earth contact required in moist earth (going therein at least 5 feet) or water.

The several lengths of the conductor must be well in contact by being screwed, if of iron; soldered, if of lead.

The upper terminal to be iron or copper rod 9 or 10 feet long, ¾ inch diameter, and 2 to 5 feet above top of highest chimney or other part of building.

It should be pointed as this attracts electricity better.

Lead roofs to be connected with conductor. (Examples given of house and ship struck.)

No building or object is known to have been struck by lightning within 50 feet of a proper conductor. But a tree has been shivered within 52 feet, so we may conclude that protecting influence extends to 50 feet horizontally in every direction from the point of conductor.

In gunpowder stores, conductors are not to be fixed to the buildings, but at (say) 12 feet away, fastened to a standard, the top being as high above the building as it can be conveniently.

No metal on sides or roof of the building is to be exposed to the lightning so as to attract it.

A TREATISE on ATMOSPHERIC ELECTRICITY.

In Chapter V., on lightning identified with electricity, the author speaks of fire-balls and the Aurora Borealis, and ascribes the formation of shooting stars to electrical action. He does not believe they come from distant space into our atmosphere, but regards them as concretions formed by a flash of lightning darting through gaseous media and atmospheric air expanded by heat, carrying metallic dust and earthy particles ejected from volcanoes, or carried up by evaporation or other causes, and diffused over an immense surface in the upper regions of the air. “The lightning carries, like a ploughshare, the accumulated matter in its progress, and, by the powerful electrical attraction thus excited, these particles will be drawn into the vortex of the lightning instantaneously; for, the lightning finally encountering an electricity of an opposite kind, an explosion ensues, and the collected mass is instantaneously fused and agglutinated, while the meteorite thus formed tumbles to the ground.... We therefore do not see the necessity of considering meteoric stones extra atmospheric.”

In this way John Murray goes on page after page, but the above will probably be sufficient notice of his work.

The following are the conditions he lays down for a good conductor:—

1. A finely pointed summit to offer an unresisting entrance.

2. A sufficient length to anticipate, as it were, the descending electricity, and receive it on its summit before it could reach any part of the building.

3. A superior conducting power in the material of the rod to facilitate its passage to the earth.

4. A sufficient thickness to prevent its fusion, which, however, will greatly depend on the resistance it has encountered in entering the conductor. And, finally

5. A safe conduction to a well or moist surface below ground.

He says: “Let the wires below ground in contact with moisture pass through a cylinder of zinc before they diverge to form the root, the copper wires will in this case always remain free from any oxidation.”

HARRIS’S LIGHTNING CONDUCTORS. REPORT to the Committee upon Mr. Snow Harris's and other Lightning Conductors.

Instances are given of ships not provided with lightning conductors being struck and damaged, whilst others lying near, and provided with conductors, were not injured. The question of lightning conductors attracting lightning considered, and evidence shown to the contrary. Lateral discharge from a lightning conductor considered. Evidence against it, if only the conductor were continuous and of sufficient size. Faraday considered that a man leaning against one of Harris’s conductors when the electricity descended would not be hurt. Proposition to place a globe of glass on the head of the mast in place of a lightning conductor considered, and the conclusion arrived at that it would do harm.

Wheatstone stated that “in the Report of the Committee of the Academy of Sciences of Paris, appointed to investigate the utility of lightning conductors, there is no instance on record of an iron rod of ½ inch in diameter being fused or even made red-hot by a flash.”

Mechanical objections to lightning conductors on ships considered and discussed. Decided that the application of Mr. Harris’s conductor tended rather to strengthen than weaken the mast and spars. Then follows a large number of letters, giving accounts of accidents from lightning to ships, &c.

Decision arrived at that on the whole Mr. Harris’s conductor is the best of those examined.

THE DIFFERENCE between LEYDEN DISCHARGES and LIGHTNING FLASHES. By C. V. Walker, Hon. Sec. Lon. Electrical Soc. London. 1842.

The author alludes to the experiments of Franklin, &c.

The distance to the lower surface of clouds, observed by Le Gentil and others, shows an average of 1000 to 2000 feet, whereas the greatest length of spark with a large machine is 3 to 4 feet.

The inductive action bears some inverse ratio to the distance.

Leaves of trees have a remarkable property of silently drawing off electricity.

He gives the particulars of a large number of experiments, with arguments thereon, to prove the theory of the difference between Leyden discharges and lightning.

Quotes examples of lightning on conductors and buildings to show that the conductor takes part only of the charge, the remainder taking other paths. Contiguous semi-insulated bodies must not be left unconnected with the lightning rod.

He quotes, with approval, the advice of Faraday, viz., to tie together with a metallic connection all contiguous readily-conducting bodies.

Cites numerous other opinions to the same effect, viz., that all metallic parts of a building should be connected with the conductor.

He sums up by stating “that the Leyden charge differs considerably not so much in nature as in degree from that of the cloud, inasmuch as the proximity of the coatings in the one case is infinitely small compared with the distance in the other,” &c.

He expresses great confidence in Sir W. S. Harris’s system for protecting ships.

THE EFFECT OF A LIGHTNING FLASH on the steeple of BRIXTON CHURCH, and OBSERVATIONS ON LIGHTNING CONDUCTORS GENERALLY. By C. V. Walker. London. 1842.

The author refers to Faraday’s experiments, as shewing instances of lateral discharge, and says, “unless precautions are taken to prevent its proceeding from a lightning conductor, that instrument literally invites the enemy within doors.”

He gives detail of the accident at Brixton, there being no lightning conductor.

The stroke did much damage to the steeple and then passed off harmlessly by the metal gutters and rain-water pipes.

One side of the steeple was drenched with wet and carried off part of the stroke.

He quotes examples of the apparently protective action of high trees.

Lofty trees near lofty buildings would materially mitigate, if not prevent, the violence of the stroke.

The accident at Brixton shows that the lightning takes not simply the shortest, but, in addition, the largest path.

Had the steeple been provided with a lightning conductor outside, passing near the clock face or the bells, or water pipe, it is more than probable that a flash would pass from it to these vicinal conductors.

If outside the tower the danger would be greater. He recommends that the metal cross on the steeple be replaced by a stone one, and that the present iron rain water-pipes be connected by copper rods or plates, which are also to be connected with the lead work of roof.

The bells are also to be connected with each other and with the conductor.

Every bolt-clamp or other piece of metal within “striking distance” of the conductor, unless in direct communication with it, is liable to cause lateral discharge.

The odour developed by lightning was, at Brixton, decidedly sulphurous, as a piece of stone which was shattered by the stroke retained the odour of sulphur distinctly for several hours.

ON THE NATURE OF THUNDERSTORMS; and on the means of protecting buildings and shipping against the destructive effects of lightning. By W. Snow Harris, F.R.S. 1843.

The backstroke may do injury, that is, a person may be killed in consequence of a flash of lightning passing between the clouds and the earth at some distance from the person.

In the Phil. Trans. for 1787, Mr. Brydone writes to the President of the Royal Society, and mentions the case of two men riding in two carts, the front one drawn by two horses, these horses and the man driving them were killed;, the man on the hinder cart and a shepherd at a distance, saw the occurrence and heard a report but observed no lightning.

A metallic screen appears to protect the interior from the action of a current, as well as from static induction.

Dr. Franklin found he could not destroy a wet rat by artificial electricity, although he could a dry one.

The first lightning conductor was erected in England at Payneshill, by Dr. Watson, in 1762.

The lightning conductor should expose a large surface, and should be united with all the great masses of metal in its vicinity. For stationary elevations the conductor should consist of solid or tubular rods or flat plates of metal. We must consider the mechanical action the lightning may produce on the conductor, as well as any possible heating action. Sir W. Snow Harris mentions that there were no signs of fusion in the fragments of the linked brass rod, at Charles Church, Plymouth, torn to pieces in 1824, or in the small pieces of the conductor at the Hotel des Invalides, at Paris, consisting of a strand of twenty iron wires, and which was smashed in 1839.

He says the benefical effect of superficial conductors appears to depend on the removal of the electrical particles further out of the sphere of each other’s influences.

“Thus we find,” says Sir W. Snow Harris, “in a variety of cases of damage by lightning that the passing charge, in striking on large expanded sheets of metal has become comparatively tranquil, and has been traced no further, whilst in striking on large masses of metal exposing but a small surface, it has assumed an intensely active state.”

He goes on to state that the resistance of the conductor must be kept as low as possible, and as neither the resistance nor the heat developed is increased by rolling the wire out into a flat surface, he argues that “there is, consequently, no disadvantage in giving a lightning rod as much superficial capacity as possible, as regards conducting power, whilst, on the contrary, the diminished intensity attendant on it is very advantageous: this effect of superficial conductors appears to depend on the removal of the electrical particles further out of the sphere of each other’s influence.”

What quantity of metal is requisite for a lightning rod? He concludes from the results of a number of accidents that “a copper rod ¾ inch diameter, or an equal quantity of copper under any other form, would withstand the heating effect of any discharge of lightning which has yet come within the experience of mankind.”

Practical deductions.—“From the various enquiries contained in the first 123 pages of this book, we arrive at the following deductions:—

“1st. Copper is the best kind of metal for a conductor.

“2nd. The quantity of metal should not be less than that represented by the section of a solid cylinder ½ inch diameter.

“3rd. The metal should be placed under as great an extent of surface as is consistent with strength, and should be perfectly continuous.

“4th. The conductor should involve in its course the principal detached masses of metal in the building.

“5th. It should be placed as close as possible to the walls which are to be defended, and not at a distance from them, and be carried at once directly into the ground.

“6th. It should be attached to the most prominent points of the building, and if the length be very considerable its dimensions should be increased.

“Lastly. In extensive ranges of buildings, all the most prominent parts should have long pointed rods projecting freely into the air, and the greater the range of building the higher they should be.

“In particular cases, in which expense must necessarily be considered, wrought iron tubing may be employed; it should not, however, be less than 2 inches in diameter, and 3/10ths of an inch in thickness.”

Insulating the lightning conductor from the building is quite valueless.

The method of fixing lightning conductors to ships is explained at considerable length.

Range over which the protecting power of the lightning rod extends.—Great doubts exists as to the answer to this question, since in many cases one portion of a building has been struck while a lightning rod in good condition existed close by.

For example, the powder magazine at Bayonne was 56 feet long, 36 feet wide, covered with thick vaulted masonry and a sloping roof with gable ends, protected by plates of lead; the gutters were also of lead, and there were the usual spouts for discharging the rain. The lightning rod projected about 20 feet above the building, and was attached to the lead of the roof by a metallic socket through which it passed, and which was soldered to one of the lead coverings. Instead of being carried, however, directly into the earth at the foot of the wall, it was turned outward at about 2 feet from the ground, and being bent at right angles, was continued on semi-insulating posts of wood into a trench filled with charcoal, distant 33 feet from the wall.

On the 23rd of February, 1829, the building was struck, the point of the conductor melted, and the leaden plates by which it was attached to the wood posts at the foot of the wall, were more or less torn and perforated by holes. No damage, however, ensued to the building in the course of the conductor. At the south-west corner, a sheet of lead covering the gable end was torn out immediately over a point where two stones of the cornice were united by an iron cramp.

Sir W. Snow Harris considers the possibility of this damage having arisen “from the conductor (in consequence of being continued at so great a distance from the building) not offering a sufficiently easy line of transit for the discharge to the earth,” but he rejects this explanation and concludes that the damage arose from the lightning striking the building in two points.

Again, the Heckingham poorhouse, although armed with eight pointed lightning rods, was struck, in 1787, at a point m, 70 feet from the nearest conductor c.

The squares at a, b, c, d, e, f, g, h, indicate chimneys to which lightning conductors were attached. The centre range was 108 feet long, the flanks each about 160 feet long: the details of the lightning conductors are not given. One portion of the lightning discharge struck one of the conductors and was carried off by it without damage to the building, one portion struck the building at the point m and also the shed at s, doing some damage, and a third portion struck the ground immediately in front of the building near a gate, G.

The ship Ætna was struck in 1830 by several heavy electrical discharges when at Corfu. These for the most part passed down a chain conductor attached to the mainmast. One of the discharges, however, struck the ship near the bow, and exploded about 12 feet above the forecastle close to the foremast, knocking people down, &c.

The Board-house at Purfleet was a lofty building with a pointed roof, well leaded and connected by lead gutters and pipes with the earth, and with wells 40 feet deep for the purpose of conveying water forced up to a cistern on the roof. It was, therefore, only thought necessary to add an iron spike about 10 feet long to the middle of the highest part of the roof. The building, however, in 1777, was struck and slightly damaged at a point 46 feet from the conductor.

Several other examples illustrating how small an area a lightning rod protects follow.

Sir W. Snow Harris further concludes that experience shows that lightning will not leap from a lightning rod to a piece of insulated or semi-insulated metal near it, although a discharge may take place between the rod and a distant metallic mass in connection with the earth, but not otherwise in connection with the rod.

He lastly considers the question, formerly much debated as to whether a lightning rod attached to a house will attract to the house a discharge that otherwise would not have struck it, and he concludes that there is no foundation for the erroneous impression that the existence of a lightning conductor can ever cause damage.

This chimney has a total height of 341½ feet (329 feet above ground), it is circular; at the top the internal diameter is 11 feet 4 inches, and the external 13 feet 10 inches; and at the bottom, internal diameter 20 feet, external 26 feet 3 inches.

Respecting the conductor Faraday was consulted, and replied as follows:—

“The conductor should be of ½ inch copper rod, and should rise above the top of the chimney by a quantity equal to the width of the chimney at the top. The lengths of rod should be well joined metallically to each other, and this is perhaps best done by screwing the ends into a copper socket. The connection at the bottom should be good; if there are any pump pipes at hand going into a well they would be useful in that respect. As respects electrical conduction, no advantage is gained by expanding the rod horizontally into a strap or tube—surface does nothing, the solid section is the essential element.^[4] There is no occasion for insulation (of the conductor) for this reason. A flash of lightning has an intensity that enables it to break through many hundred yards (perhaps miles) of air, and therefore an insulation of six inches or one foot in length could have no power in preventing its leap to the brickwork, supposing that the conductor were not able to carry it away. Again, six inches or one foot is so little that it is equivalent almost to nothing. A very feeble electricity could break through that barrier, and a flash that could not break through five or ten feet could do no harm to the chimney.

“A very great point is to have no insulated masses of metal. If, therefore, hoops are put round the chimney, each should be connected metallically with the conductor, otherwise a flash might strike a hoop at a corner on the opposite side to the conductor, and then on the other side on passing to the conductor, from the nearest part of the hoop there might be an explosion, and the chimney injured there or even broken through. Again, no rods or ties of metal should be wrought into the chimney parallel to its length, and therefore to the conductor, and then be left unconnected with it.”

In answer to some further inquiry, Professor Faraday again wrote:—

“The rod may be close along the brick or stone, it makes no difference. There will be no need of rod on each side of the building, but let the cast-iron hoop and the others you speak of be connected with the rod, and it will be in those places at least, as if there were rods on every side of the chimney.

“¾ rod is no doubt better than ½ inch, and except for expense I like it better. But ½ inch has never yet failed. A rod at Coutt’s brewery has been put up at 1½ inch diameter—but they did not mind expense. The Nelson column in London has ½ inch rod, ¾ is better.

“I do not know of any case of harm from hoop-iron inclosed in the building, but if not in connection with the conductor, I should not like it; even then it might cause harm if the lightning took the end furthest from the conductor.”

The following paragraph states what was done:—

“The electric conductor stands 6 feet above the iron top-plate, ?-inch round copper, made fast to stone and brickwork with 7?-inch copper holdfasts let 4 inches into the masonry or brickwork, with a head on the inside and an eye on the outside to receive the rod as it was carried up. By these holdfasts an ascent can easily be made to the top by a small tackle suspended to the holdfasts. The conductor is metallically connected to all the ironwork on the stalk—the plate on the top, projecting cope, malleable iron hoops, bolts on the top of stone pedestal, and also the ascending chain. The rod descends into a well about 10 feet from the foundation, and is immersed about 8 feet deep in water, and the end turned up 2 feet in a horizontal direction, and flattened.”

PAPERS relative to SHIPWRECKS BY LIGHTNING, as prepared by Sir Snow Harris, and presented by him to the Admiralty.

Number of merchant ships destroyed by lightning, loss to the country. Application of lightning conductors to ships in 1820. Mode of applying them. Mechanical difficulties; how overcome. The saving to the Exchequer which has resulted.

Long account of various ships in the Royal Navy not provided with lightning conductors, struck by lightning and damaged. Loss of life and injury that has resulted. Long account of ships provided with lightning conductors, and so preserved.

Sir Snow Harris states that “although his system of lightning conductors ought to guard against all those violent and regular shocks of lightning falling within the ordinary experience of mankind, it is not to be expected that the system could guard against every possible kind of atmospheric electrical discharge, be the circumstances what they may, such as thunderbolts, fire-balls; nor is it expected that it should guard against meteorolites, or against sweeping electrical action mixed up with convulsions of nature; nor can it quiet those minor electrical effects producing electric glow; nor can it always obviate that tremendous concussion and expansion of the atmosphere in cases in which a thunder-cloud discharges its lightning in a dense explosion on the masts, and which may rupture, or mechanically tear to pieces, frangible matter.”

M. Duprez refers to the Report of a Committee of the Institute of France. (Vide Comptes rendus, 1852–6.)

He divides the subject into the following heads:—

1. The frequency with which lightning rods are struck.

2. Their terminal points and the effects of the stroke on them.

3. The conductors and their ground connections.

4. The protective power of the lightning rods.

1. Concerning the frequency with which lightning conductors are struck by lightning.

The author cites 144 cases of lightning rods having been struck. Of these seventeen were struck two or three times, so that the total number of electric discharges on them was 168, as far as recorded.

But very many cases are not recorded at all, e.g., from 1793 to 1813 only two cases were noted. The great number of lightning rods struck would seem at first to support the idea that they attract lightning.

But we must compare the number of rods struck with those fixed, and we find from a communication made in 1777 to the Academy of Berlin, that, even then, a large number were fixed to the most important edifices of N. Italy and England.

The same in 1784 to those in the ports of France and to the ships in the said ports.

In 1794 the fortresses of Russia were ordered to be so protected.

In 1769 there were 166 edifices in Hamburg alone, and 104 in its environs, with conductors.

If the number of conductors were so great in the last century, we must conclude that the number of those struck must be very inconsiderable as compared with those fixed.

In Hamburg, e.g., not one rod is recorded as having been struck.

In 1785, Ingen-Housz reports that of all the lightning rods placed by his direction on the Austrian powder magazines and other buildings only one had been struck.

In 1772, Franklin wrote, that during the twenty years in the course of which lightning rods had been fixed in America he knew of five cases only in which these rods had been struck.

Sir W. S. Harris reports in 1854, as the results of twenty-two years’ experience, that the number of vessels struck unprotected by lightning rods, as compared with that of vessels protected by his plan, was as three to two.

The above show that the idea of danger from lightning rods is not well founded.

Besides which it must be remembered that they are frequently placed in the most exposed positions, e.g., of the 144 rods struck, seventy-four were on ships, and fifteen others on buildings which had been struck before.

One would think that the number of terminals placed on a building would diminish the chances of their being struck, but it does not seem to be so; e.g., twelve buildings in the first list had many terminals communicating with a common conductor or different conductors.

Yet the lightning struck, with explosive effect, one or other of the rods of these buildings.

And in each of two cases the lightning struck at once the three rods fixed to a building.

Of the 144 cases above cited:—

74	were to lightning rods fixed on ships
30	were to lightning rods fixed on towers
9	were to lightning rods fixed on powder magazines
31	were to lightning rods fixed on ordinary buildings.
144

In forty-four cases where one of Sir W. S. Harris’ conductors was fixed to each mast of a ship, the mainmast was struck twenty-seven times; the foremast was struck fourteen times; the mizen was struck twice; both the main and foremast twice.

2. As to the points of the lightning rods struck, and the effect produced on them.

(Sir W. Snow Harris’s system as adopted in the British Royal Navy since 1830 is described. They are formed of bands of copper let into the masts. They have no upper terminals or points, and fifty-five are included in the list already quoted of 144 lightning rods struck.)

Of the eighty-nine cases remaining in the list, only fifty-one are recorded as having their upper terminals ended with points.

Of these, thirty had their points melted to a greater or less extent; six of them were of copper or brass; five were of copper gilt or iron gilt; one was of brass silvered; and four were of platinum. The others are not distinctly described, and the sizes seldom given.

One of brass was 25·4 centimetres (c. 10 inches) long, and 5 millimetres (?th inch) diameter at its base, and was melted for ¼th of its length.

One of copper was 24 centimetres (c. 9½ inches) long, and 9 millimetres (c. ?rd inch) diameter at base, and was almost all melted.

One of platinum was 8 centimetres (c. 3 inches) long, and 1 centimetre (c. ?rd inch) diameter at base. This was melted for a length of 5 or 6 millimetres (c. ?th inch.)

It results from the above facts that the points of the lightning rods have been much too slender.

The Institute of France recommends, therefore, for the points 2 centimetres diameter (c. ¼th inch) at base, and only 4 centimetres (c. 1½ inches) high, with an angle of opening of 28 to 30 degrees.

It has been urged, especially in Germany, against the employment of pointed upper terminals that these points are fused by the lightning, this fusion being regarded as dangerous on account of its action on inflammable substances near.

As to this, the author cites three cases of buildings set on fire, though protected by lightning rods. But the precise cause of the fire was not ascertained.

Several observations show that the melted metal trickled down the side of the lightning rod.

At Strasbourg the metal was pressed down on one side, and had bent like wax softened by heat. At other times the lightning disperses the melted metal in all directions. (Examples quoted.)

With these facts before us we cannot altogether deny that some danger may arise from the fusion of the metal at the point of the terminal. But this danger can be much lessened, if not removed, by adopting the size, etc., of the lightning rods recommended by the Institute of France.

Besides fusion, the points sometimes show distinct traces of mechanical action caused by lightning.

The author quotes six examples of this where the points had been curved.

This shows the necessity of strengthening the points of the upper terminals. The curvature arises, probably, from the points being much heated by the lightning, and acted on by the wind.

One case is noted of a point which had the appearance of having been struck violently by a hammer.

Also of one in which the base of a point, where it was screwed to the rest of the upper terminal, was split for a length of 11 millimetres (c. ½ inch).

Also of a platinum point screwed on the upper terminal (copper), and retained by a pin, where the stroke tore away the pin, the point falling intact at the foot of the lightning rod.

3. Of Conductors of lightning rods struck, and their contact with the ground.

The author refers to forty-one cases of lightning rods struck when not on Harris’s principle.

Of these, 5 were of copper bands soldered together; 5 were of copper wire either as rope or chain; 1 was made of bands of sheet iron; 11 were of bars of iron joined by screws or by solder; 3 had pieces of lead between the parts where they were screwed together; 3 were of simple iron wire, or of rope or chain of iron wire; 3 were of iron joined together by hooks; 12 are described as chains (metal not specified); 1 is described merely as a conductor.

The dimensions of the above are seldom given.

The largest bands reported are 16 centimetres (c. 6¼ inches) in width.

The largest bars reported are 55 centimetres (c. 2¼ inches) in width and 15 centimetres (c. ½ inch) in thickness.

The description of the earth connection is also imperfect.

Of eighty-nine lightning rods described as struck, only twelve are noted as having their ends in running water or wells, and one in damp soil.

Fifteen simply entered the ground, it being noted expressly of six of these that it was dry.

In three cases were the lightning rods were struck the author found that the part at the base and in the damp earth had terminated in a plate of lead, protected above the ground by a wooden enclosure.

Three conductors of ships did not communicate with the sea.

Twenty-three cases are noted of ordinary conductors (not on Sir W. S. Harris’s principle).

The lightning melted, or reduced almost to powder, three.

The first was on a house, and was of copper wire, the diameter not known, ending with a chain of iron buried in the earth.

The second was on a ship’s mainmast, and was of iron wire 6 millimetres (c. ¼ inch), diameter, 46 centimetres (c. 18 inches) long, folded at their extremities, and united by rings.

The third (also to a ship) was a rope of three strands formed in the whole of 60 brass wires, each being one half to two-thirds of a millimetre thick.

The two last conductors had their ends in the sea.

The parts of these conductors, in place of being soldered or screwed together, were joined merely by hooks and rings like a surveyor’s chain. Evidently a bad form as their contact is imperfect.

In three other conductors, whose different parts were screwed together with lead between them, the stroke melted the lead.

This shows the danger of lead from its fusibility, in addition to its less conducting power.

The author gives examples of this, wherein a leaden pipe, 8 centimetres (c. 3¼ inches) external diameter, and 13 millimetres (c. ½ inch) thick, was melted.

He quotes Arago as calling attention to the importance of the form of the bends in conductors, abrupt bends being dangerous.

Two examples are quoted to prove this, the conductors having been broken by the lightning stroke at a sudden bend.

To provide lest the lightning, after having struck the lightning rods, should abandon them for larger masses of metal near them, these masses should be made to communicate with the conductors.

Cases are cited where the lightning quitted the conductor and struck metallic bodies near. Also, in respect of painting conductors, the author quotes a case where part of a bell wire adjoined a lead pipe which communicated with the conductor. Part of the wire was painted in oil colour, the other part not. The latter was melted, the first not, but the paint (though otherwise uninjured) had ceased to adhere to it.

Three examples are cited of danger from conductors ending in watertight tanks.

In one case the stroke broke the conductor.

In another it left the conductor and injured the building.

In the third it merely melted the point of the upper terminal.

Nevertheless, it often happens that the lightning, in spite of imperfect communication with the earth, disperses itself inoffensively.

Out of fifteen cases of lightning rods struck, in which the conductors were simply buried, more or less, in the soil, they carried off the stroke in eleven without the buildings being injured, or any trace being left of it, except that the ground was upheaved where the latter was too dry.

The French Institute, in their report on the protection of the Louvre, considered it necessary to employ, under certain circumstances, a conductor with two branches, the one descending into a subterranean source of water, the other communicating simply with the surface of the earth.

On the other hand, Arago thought that conductors need not enter the ground, but communicate only with a metallic surface lying on the ground.

This view is confirmed by the cases which the author mentions where the surface of the earth being wetted by rain formed a conductor.

Nevertheless, the two branches are desirable, in case one should fail.

Fifty-five conductors on Sir W. S. Harris’s system are recorded as having been struck, but the damage was quite trivial.

Two electrical phenomena are to be noted as sometimes occurring when a lightning rod is struck.

First, when a conductor is formed of metallic plates a peculiar noise is heard like water pouring on a fire.

Second (independently of the form of the conductor), electric sparks are emitted from bodies near. The author cites example at Berne, 1815.

4. Protective agency of lightning rods.

Out of 168 cases of lightning rods struck (vide page 91) there are only twenty-seven (c. ?th) in which the buildings or ships have not been preserved, and of this sixth many of the conductors were imperfect; e.g., four terminated in earth which was unusually dry, and two of them were of insufficient size.

Another was formed of pieces having their ends hooked.

Two conductors ended in watertight tanks.

Another was in the form of a surveyor’s chain, the parts not being, consequently, in close contact.

Others were badly jointed, or had imperfect communication with the ground or with the sea.

In two cases the stroke broke the conductor at points where its direction was abruptly changed.

In two other cases the lightning left the conductors struck, and fell upon buildings near without causing damage to those on which the rods were fixed.

In the instance of a lightning rod fixed to the mainmast of the Jupiter (1854), the conductor was made of sixty brass wires, one half to two-thirds of a millimetre (0·02 inch) thick, and was broken by the stroke into thousands of pieces. The Institute Committee concluded that the lightning was not conducted by all the wires of the conductor. Those which it followed were insufficient to transmit it; some were melted, some broken. The Committee recommended, therefore, that each metallic wire be tinned separately at the extremity of the conductor, and soldered thereto for a length of about a decimeter (c. 0·4 inch), so as to form a metallic cylinder.

In the last six cases the particulars of the lightning rods are not given sufficiently to show the cause of their failure, but five are described as being of chain or ropes of metal wire.

It results from the above facts that when the lightning rods have proved insufficient protection, their failure has been owing to defects in their construction; it is rather surprising to find how well buildings and ships have been protected, even when the lightning rods have not been well constructed.

In every one of the fifty-five cases where Sir W. S. Harris’s rods were fixed they have protected the ships, except that not having points some slight damage has sometimes occurred to the tops of the masts.

This shows their superiority over ropes or chains.

Arago thought that lightning rods were protection against ordinary lightning, but not when it assumed the form of fire-balls. The author cites several examples to show that this opinion was not well founded.

He considers a perfectly constructed lightning rod to be a perfect safeguard.

But he adds that the lightning stroke produces electric disturbances in its vicinity, although the building be intact.

He cites an example of this in respect of a prison whose inmates (300) experienced a great enfeebling of their muscular power during some seconds.

Very few records exist relating to the area of action of lightning rods, and the elements for determining their protective power are slight. The author gives a table showing the heights of points, horizontal distances, &c., in certain cases, and cites four instances of ships whose foremasts were struck although the mainmasts had lightning rods, and one where the mizen was struck though the fore and mainmasts were protected.

TABLE GIVEN BY M. DUPREZ.
	In Metres.				In English Feet.
	1st Case.	2nd Case.	3rd Case.	4th Case.	1st Case.	2nd Case.	3rd Case.	4th Case.
Length of upper terminal, or height of point above that portion of the building on which the upper terminal was fixed.	1·5	3·4	1·5	2·3	5	11	5	8
Vertical height of point above the place struck.	1·5	7·6	6·7	71·2	5	25	22	232
Horizontal distance of place struck from the base of upper terminal.	15·2	7·3	17·4	59·9	50	24	57	197

These instances show that we should be misled in considering, as being protected, a circular space whose radius was double the height of the lightning rod.

The protected radius appears to be only equal to double the simple height of the upper terminal above any required point, and reckoned horizontally from a point vertically under the conductor.

[It will be observed that M. Duprez here contradicts himself in two consecutive sentences, and in a subsequent part of his work (p. 30) of the Memoir, he again says: “Aucun des cas indiquÉs dans le numÉro prÉcÉdent n’infirme la rÈgle gÉnÉralement admise, savoir que la sphÈre d’action d’un paratonnerre s’Étend, dans toutes les circonstances, À un espace circulaire d’un rayon Égal au double de la longeur de la tige, c’est-a-dire de la hauteur de la pointe au-dessus de la partie du bÂtiment sur laquelle la tige est fixÉe.”

But the table given by M. Duprez gives two instances in which the stroke fell within the radius of once the height.—Ed.]

RESUMÉ.

In the paragraphs which the author numbers 1, 2, 3, 4, and 6, he refers to former statements as to the proportion of lightning rods struck, &c. (Vide page 91, &c.)

5. There being several terminals on an edifice does not seem to diminish the chances of each being struck.

7. In vessels, when the three masts have lightning rods, the mainmast is most frequently struck.

8. Refers to Sir W. S. Harris’s lightning rods as being without terminal rods or points.

9. The points of ordinary lightning rods have been made too slight.

10. Out of fifty-one cases of lightning strokes, thirty points have been more or less melted; and the fusion is not without danger to the buildings.

11. The lightning often leaves traces of mechanical action more or less decided.

12. Refers to defective constructions of ordinary lightning rods.

13. Lead plates in conductors composed of bars joined together are dangerous.

14. So are abrupt bends.

15. Conductors should communicate with the masses of metal near.

16. And must not end in watertight tanks. But

17. Conductors often protect buildings, though the ground connections are imperfect.

18. It is well for a conductor to have two branches, viz., one in water, and the other on the surface of the ground.

19, 23. Refers to the complete efficiency of Sir W. S. Harris’s conductors.

20. Mentions the noise, electric sparks, &c., given off during a stroke, as before stated (page 95).

21. Mentions the efficacy of lightning rods generally.

22. Their failure being owing to defective construction.

24. There is no proof that the electricity being in the form of a ball has been the cause of any conductor’s inefficiency.

25. The lightning rarely bursts on a building or ship without striking the lightning rod placed on it. Exceptions have, however, occurred in ten cases, as here described. But

26. None of these instances invalidate the rule generally admitted, that the protective action of the lightning rod extends, under all circumstances, to a circular space whose radius is equal to double the length of the upper terminal, i.e., the height of the point above the part of the building on which the upper terminal is fixed.

ON ATMOSPHERIC ELECTRICITY.

This is a pamphlet of seven chapters, and fifty-seven pages, written to ventilate the author’s own notions of the nature of electricity and its production in the atmosphere. He considers electricity to be two fluids of a species of substance, consisting of separated subsidiary atoms. “Electricity is comparable to a flying bullet; the vis viv of the bullet is like electrical intensity, and the mass of the bullet answers to the quantity of electricity.” What the subsidiary atoms are like he does not say.

Chapter I. is a fair resumÉ of what is known of the electricity evolved by the friction of wet steam against solids in the hydro-electric machine. He agrees with Faraday that the cause of the evolution of electricity by the liberation of confined steam, is not evaporation, but the friction of the water particles against the sides of the jet-piece or orifice. Pure gases do not excite electricity; but impure air, when compressed does, from the friction against the orifice of those particles of water which are suddenly condensed by the cooling influence of the expanding air.

Chapter II. is an attempt to show that electricity is evolved by the friction of “gaseous matter” against water, or vice versÂ. Ordinarily the issuing vapour in the hydro-electric machine is positively, and the boiler negatively, electrified; but cases occur where this is reversed. According to the author, water by friction against gaseous matter or air, becomes positively electrified.

Chapter III. applies this theory to thunder clouds, which are formed by the rapid inter-mixture of masses of the atmosphere thrown into circulation by heat. There are some capital descriptions of thunder clouds. They are often accompanied by whirlwinds, and always by rain. It is the friction of the whirlwind on the drops of rain that developes electricity—the rain being positively, and the air negatively, electrified. Hail is due to the ascending current of air carrying the drops of water to the region of snow and frost! His notions are somewhat hazy, thus:—“Much of the positive electricity is conveyed to the earth by the lightning; but the corresponding negative electricity from being carried upwards with the vertical wind, cannot so easily escape to the earth, so that the storm cloud contains, on the whole, more negative electricity than positive electricity” (p. 38.)

Chapter V. contains the author’s explanation of fire-balls, which he supposes to be a “glowing discharge,” preparatory to the final spark or flash of lightning. “Probably most of the shooting stars are merely electrical fire-balls high up in the atmosphere”(!)

Chapter VI. is devoted to the Aurora Borealis, which plays about the magnetic pole, and is an electrical phenomena of the upper strata of the atmosphere; and Chapter VII. is an attempt to explain the auroral light as “probably produced by the collision of the subsidiary atoms when they are in the act of electro-apposition.”

The pamphlet is said to be a condensed account of the discoveries of the author in matters connected with atmospheric electricity—discoveries which were described in papers handed to the Royal Society, but which that Society did not read. The Royal Society were wise.

LE COUP DE FOUDRE DE L’ILE DU RHIN PRES DE STRASBOURG. Par M. F. Hugueny. 4to. Paris. 1869.

A very full account of an accident by ball lightning. The facts are set out as clearly as possible, the authority is given for every statement, and most carefully engraved plans and engravings are given of all the necessary details. It does not bear upon the question of lightning conductors except in that it shows that a discharge of globular lightning traversed a horizontal distance of 919 yards, passed in front, but below the top, of a building which had three good conductors upon it and struck a chestnut tree, which was by no means the highest tree in the locality.

DIRECTIONS FOR CONSTRUCTING LIGHTNING RODS. From “Essays on Meteorology,” by Professor Joseph Henry.

1. The rod should consist of round iron, of not less than ¾ of an inch in diameter. A larger size is preferable to a smaller one (ordinary gas pipe may be employed). Other forms of rod, such as flat or twisted, will conduct the lightning, and in most cases answer sufficiently well. They tend, however, to give off lateral sparks from the sharp edges at the moment of the passage of the electricity through them, which might, in some cases, set fire to very combustible materials.

2. It should be throughout its whole length in perfect metallic continuity, either by screwing the parts firmly together or by welding.

3. The rod should be covered with a coating of black paint.

4. It should be terminated above with a single point, the cone of which should be encased with platinum not less than 1/20 inch in thickness.

5. The shorter and more direct the rod is in its course to the earth the better; acute angles should be avoided.

6. It should be fastened to the house by iron eyes, which may be insulated by cylinders of glass; this, however, is not absolutely necessary.

7. The connection to the earth should be as perfect as possible—in cities nothing is better for this purpose than to unite it to the gas or water pipes. When a connection cannot be formed in this way the rod should terminate in a well containing water, or if this is not practicable it should terminate in a plate of iron, or some other metal buried in moist ground. It should, before it descends to the earth, be bent, so as to pass off nearly at right angles to the side of the house, and be buried in a trench, surrounded with powdered charcoal.

8. The rod should, in preference, be placed on the west side of the house, and on chimnies where a current of heated air ascends during the summer season.

9. A single rod may be placed on small houses, and its elevation should be at least half of the distance to which its protection is expected to extend.

10. Metallic roofs should be united with the lightning rods.

11. As a general rule, large masses of metal within the building, particularly those which have perpendicular elevation, ought to be connected with the rod.

ON LIGHTNING AND LIGHTNING CONDUCTORS. By W. H. Preece, Mem. Inst. C.E.

The author refers to the Escurial having been on fire seven times—four of them certainly from lightning; yet no lightning conductor is fixed even now.

The average deaths from lightning in England are eighteen per annum; in France, ninety-five.

From January 1, to July 31, 1872, 9·26 per cent. of instruments, of different forms, used in the telegraph offices, were injured by lightning.

Electricity is force, not matter, and Current is a well-defined term which implies a transference of electricity from one place to another.

Thunderstorms differ only in degree from the phenomena which cause the ordinary snapping sparks from the machine.

In any case there must be two conducting masses in opposite electrical states, separated by a non-conductor or dielectric.

The light is the effect of the discharge, and is simply incandescent matter. It indicates the path of the discharge and nothing more.

Death by lightning is painless.

Potential is that function of electricity which determines its motion from one point to another.

The path of electrical discharge is prepared beforehand by induction.

The particles of air, &c., are in a state of “tottering equilibrium.” A moving ship, a man on horseback may destroy this, and we have a discharge with all the effects of light, heat, and mechanical energy.

It is very doubtful whether thunder-clouds are themselves the sources of electricity, producing thunder and lightning; they are more probably, mere accumulators as the coatings of a Leyden jar.

Clouds have been known to be absent during a discharge.

Moreover, the charge of a Leyden jar exists not in the coatings but in the dielectric separating them.

So the discharge exists in the air and not in the clouds.

Sheet lightning is a mere reflection of forked.

Evidence proves that some such phenomena as ball or globular lightning exists, and an explanation of it has been given by C. Varley.

Discharge is invariably through the line of least resistance. It may be through metals, bricks, trees, animals, and not always in a single track; it is often divided into two, three, or even four lines.

Thus, an electrical discharge in air, is simply a discharge between two electrified conductors, of such different potentials as to break the resistance of the dielectric separating them.

There is nothing hidden, mysterious, or unknown in it.

A ship is a prominent object; generally a conductor, and reduces the line of resistance between the sea (inner coating) and the cloud (exterior coating of the condenser) determining discharge.

Trees, buildings (except tall spires, &c.) are less prominent.

The effects of lightning experienced on telegraph wires, poles, and instruments by direct discharge are less numerous than those by induction, and seldom destructive.

There were only two cases in the past season where line wires (No. 8 iron, diameter 0·170 inch) were absolutely fused.

Accumulation of a charge upon a cloud converts it into a powerful inducing body.

It induces in the wire an opposite electrical state. Discharge takes place. The cloud suddenly loses its coercive power. The wire recovers its neutral condition, and produces a powerful current in opposite direction.

Wires are affected although buried two feet underground. Unprotected poles are often destroyed. In one case, twenty successive poles were so.

Instruments have had their cases burst out, wood-work has been burnt, and the wires of electro-magnets, &c., been fused.

Clouds are not perfect conductors, so do not part with all their discharge at once. There may be several successive discharges.

Protection.—Sir. W. S. Harris’s system approved.

Houses.—Unnecessary expense is often incurred in protecting them.

A warm flue, terminating in a metal grate, is a dangerous conductor, as it ends in the room and not in earth: hence so many accidents indoors. A lightning conductor should expose a prominent metallic point, and offer a path of little or no resistance thence to earth.

Hitherto expensive plates or ropes have been used for this. But the author thinks galvanized iron wire ¼ inch diameter amply sufficient for any dwelling house.

Telegraphic poles, protected by lightning conductors of No. 8 wire (½ the above size), have never been injured.

In one case, fifteen per cent. of unprotected poles have been struck.

But no case of damage has occurred for many years since the poles were earth-wired. The cross-arms are often damaged as far as the earth wires, never below.

The author can conceive no case in which ½ inch standard galvanized iron wire is not ample.

The conductor should be solid and continuous from the gilded or platinum point to the ground.

Joints should be well soldered. Chains and linked rods should not be used.

Earth connections should be formed with iron gas- or water-mains, or be several feet in coke, or in a well.

Each conductor should make a separate earth.

All masses of metal in the line of probable discharge are to be connected with conductor.

Conductors should be examined periodically, they should not be insulated, nor be near soft metal gas-pipes, nor bent in acute angles.

The area of protection appears to be that of a cone, whose radius is equal to the height of the conductor.

One conductor is enough for small houses, but each stack of chimneys should have one in connection with the main conductor.

Lead roofs and iron pipes are easily made into protectors for buildings.

Details given for protecting telegraph apparatus.

The telegraph companies abandoned the use of protectors. The Post Office re-introduced them with good results. The Indian telegraph apparatus is protected, and accidents scarcely ever occur.

Prevention.—Points prevent the accumulation of charges. But with very tall conductors—as to spires—a current results constantly in one direction, producing electrolytic action and destruction of conductor, as proved by one at Llandaff Cathedral.

So earth should be made with large masses of metal, as gas- or water-mains.

Galvanised iron fastenings should not be used to secure copper conductors to buildings, as galvanic action would be set up.

Appendix.—Letters quoted from Mr. Latimer Clark and Dr. Faraday as to damage to underground wires from lightning.

The Discussion on Mr. Preece’s paper was conducted by Prof. Abel, Capt D. Galton, Mr. G. J. Symons, who referred to Dr. Franklin’s suggestion as to cold fusion, Prof. Ayrton, who entered at length into the system of prevention used with the Indian telegraphs, Sir W. Thomson, and Mr. Latimer Clark.

Mr. Preece replied, more especially alluding to the phenomena of fire balls.

LIGHTNING RODS AND HOW TO CONSTRUCT THEM.——By John Phin, C.E. New York. 1873.

The author is not an electrician nor a patentee, but the Editor of an engineering paper called the Technologist. The book is written chiefly to counteract the machinations of a great nuisance in the United States, called the “lightning rod man.” The Author thinks a good rod as important as a fire insurance policy. Every case of injury that he has examined was due to defective rods, or to the absence of them. The lightning rod is an American invention. He mentions several cases of marked immunity from accident due to proper conductors, notably St. Paul’s and the Monument, London; the Cathedral, Geneva; and St. Mark’s, Venice.

The lightning rod should form the path of least resistance, and it may be of iron or of copper. If of iron he prefers a flat bar 1 inch by ¼ inch, weighing 13 ounces per foot, or No. 00 copper, weighing 6½ ounces per foot. He also advocates copper rope.

He thoroughly believes in the conduction through the mass of the metal, and quotes (p. 12) several experiments in support of that view.

He believes in a good earth and in connecting all waterspouts, eaves, gutters, and metal work generally with the earth and with the conductor; he thinks one good rod enough, and sees no reason why lightning rods should not be painted, indeed, thinks it better to do so, for they become less unsightly; he has no faith in points, nor in gilding, or platinising; he recommends instead cast iron caps to chimneys; he discards insulation as absurd, and suggests that rods may be tacked, or stapled, or strapped to buildings, although he prefers staples; recommends strongly that wet earth should be reached, and that as large a metal surface as possible should be exposed to the ground and embedded in coke; he does not like any connections with the gas pipes.

He suggests that iron conductors may be welded or have merely butt joints, but recommends solder with copper, after being bound with fine wire.

He adduces the fact that Mr. Brooks, of Philadelphia, measured the resistance of three rods attached to three buildings that had been damaged, and found the average to be above the resistance of one hundred miles of telegraph wire.

TRAITÉ DES PARATONNERES, &c. Par A. Callaud. Paris. 1874. Royal 8vo.

This work consists of 171 pages. It commences with a short history of the subject, which occupies the first chapter. The remaining nineteen chapters treat successively of the collecting points and their mode of action; the conducting rods and the methods of attachment to different classes of building, and their connection with the earth, with concluding observations.

The second chapter treats of the height of conductors and the area protected, in which he follows the usual rules, and recommends lofty rods, their office being not only to safeguard the building, but to withdraw electricity silently from the air and thus prevent strokes of lightning or diminish their violence.

In Chapter III., after citing the opinions of many other writers, he strongly advocates protectors furnished with sharp points of platina, or some inoxydisable metal, securely screwed and soldered on to copper rods, and condemns points of iron or copper. Throughout the work he treats cost as a secondary consideration and considers it false economy to spare any expense necessary to ensure the thorough perfection of the whole system.

In Chapters IV., V., and VI. he gives drawings of connections and of various forms of weathercocks.

In Chapter VII. he recommends multiple points, especially in mountainous countries and where storms are prevalent. He also points out that many buildings are naturally protected by the metal roofs and ornaments belonging to them. So long as these are connected with the ground, he prefers that the projecting rod should be of round iron of considerable length and in one piece, and the conducting cable should wind round it as a collar, and be strongly attached to it by set screws and soldering. He does not advise that all the masses of metal within a building should be connected with a conductor, especially if they are in proximity with human beings, but with a well-made conductor he considers it safer to leave them isolated. (Chapter IX.)

For the conductor he recommends Gay Lussac’s construction, viz., a rod of iron about ? inch square, carried by iron supports, or a twisted cable of iron wires having a diameter of ? inch to ¾ inch, well tarred or galvanised, 6 or 8 feet from the soil these are securely united to an iron bar ? inch to 1 inch diameter. If of copper they may be smaller. Has seen rods of copper of ? inch effectually protect churches, but regards this as a minimum size for a length of 80 feet and ¾ inch as a maximum. The single wires of the cords may have a diameter of 1 millimetre; the joints are made by splicing the strands together and soldering them. (He recommends conductors of straw in some cases for country use. Chapter X.)

The conductor is led along the ground in a channel of half drain tiles, surrounded with coke and terminates in a copper grapnel embedded in a basket of coke. (Chapter XIII.)

Chapters XIV., XV., and XVI. gives details of the construction of lightning conductors for tall chimneys, powder magazines, and ships.

In Chapter XVIII. he gives numerous examples of the utility of conductors, and in Chapter XIX. he gives a resumÉ of his instructions, again insisting on the perfect continuity of the connections and the perfection of all the parts; these instructions are also embodied in a note read before the Academie des Sciences, in 1862, a copy of which is given at page 167 of M. Callaud’s work.

BLITZABLEITER-ANLAGEN. PROF. C. ZENGER’S SYMMETRISCHE BLITZABLEITER. C. Korte and Co., Prague.

This is really a trade circular, but it gives, in a compact form, the considerations which have induced Prof. Zenger to propose his new system, and a description of the mode in which it is carried out. In the first place it may be well to reprint from the Meteorological Magazine, Vol. VIII. (1873), page 155, the report of the paper read by Prof. Zenger at the British Association Meeting.

PROF. ZENGER, ON THE ACTION OF SYMMETRICAL CONDUCTORS AND LIGHTNING CONDUCTORS.

Professor Zenger read a paper, on this subject, illustrating it with the well-known experiment in physics of placing two insulated hemispheres of brass plate in contact with another insulated sphere of brass. If the former were charged with electricity and removed from the inner brass sphere, there was found no trace of electricity on its surface. The electricity was shown to be accumulated on the surface of the outer spherical conductor, with equal tension in every point of the surface. Professor Zenger showed that if the outer hemispheres were replaced by two circular wires, no action whatever in the inner conductor was found. He said it was easy to see that this simple experiment might prove useful in regard to the construction of electric apparatus and of lightning conductors to protect buildings, and even whole cities, from the destructive action of atmospheric lightning. He had, therefore, endeavoured to ascertain the effects if any other form of a symmetrically-arranged conductor were used, instead of a circular form. In the first instance, he had tried the parabolic wires joined to the electroscope; next, a rectangular wire with five different openings. If placed exactly in the middle of the rectangular wire, no action was observed; if placed eccentrically, however, there was small but increasing action; and if he placed a needle or another sharp-pointed instrument between the protecting wire and the electroscope, he still better observed the different action produced by placing the electroscope in an eccentrical position. He therefore thought that it was possible by symmetrical wires placed on buildings, or over whole cities, so to procure an entire protection from atmospherical electricity. If the electric clouds should even enter between the objects protected and the protecting wires, their activity would be greatly diminished, for the wires would become immediately charged, and nearly all the electricity accumulated on their surface without any danger to the protected buildings.

Mr. Glaisher, who had taken the chair in the temporary absence of the president, said their thanks were due to Professor Zenger for his communication upon a subject so important. What they wanted to know was the distance at which buildings were protected by a lightning conductor, and Professor Zenger’s assertion that the sections of a globe were as effective as the whole globe itself, would be an important addition to scientific knowledge if proved to be so.

Professor Clerk-Maxwell, who said he had paid some attention to the subject of shielding bodies from electrical action by means of the wire, feared that the form that Professor Zenger had given them would be rather difficult to work out mathematically.

Professor Zenger said that the correspondent of the Engineer newspaper had just informed him that the instrument hut of the Atlantic Telegraph Company at Valencia was protected by wires on the principle he had just mentioned, and the plan of protecting the hut had been devised by Mr. Cromwell Varley.

We now pass on to Messrs. Korte’s paper, which refers entirely to the application of this symmetrical principle to buildings. They begin by claiming that Prof. Zenger’s system is the only one based upon scientific investigations and practical experiments, and that although far better than the primitive arrangements generally adopted it costs no more. They urge that the conductors should be symmetrically arranged, and yet they say that they should lead to the side of the house most exposed to the weather. They recommend that the upper terminal should be a long oval of gilt brass, something like a blunt spear-head, and that, in ordinary cases, a single copper rod of 0·20 inch diameter (not a rope of that size) will be sufficient; it is to be taken through porcelain insulators, and the earth terminal is to be a copper plate nearly ¼ of an inch thick, buried from 6 to 9 feet deep in coke.

PROTECTION OF LIFE AND PROPERTY FROM LIGHTNING. By W. McGregor. Bedford, 1875. 8vo. 43 pages.

Mr. McGregor does not give any new facts in connection with lightning, but discusses the theory and action of conductors, and quotes numerous opinions from other writers, with practical suggestions and precautions to be observed in fixing conductors.

Among the principal opinions adduced are the following:—

1. Professor Jenkin’s statement, that if a conductor be armed with a point, the electricity passes into the air rapidly in times of excitement by induction, and so equalises the tension of the surrounding atmosphere as to mitigate, or, in some cases, to prevent the discharge of lightning.

2. De la Rive’s observation that a slight break of continuity in a conductor is filled by a succession of brilliant sparks during a storm, though there be no lightning; that blunt points or balls are equally effective when struck, but are more usually accompanied by explosion than by continuous discharge.

3. The opinions of De la Rive, Dr. Mann, and Preece, that a conductor practically protects a conical space—of which the radius is about double the height—and that the conductor should therefore extend to some height above the building.

4. Ganot’s opinions that a conductor should terminate in a point or points, have sufficient sectional area, be thoroughly connected with the earth, and be connected with lateral metallic surfaces of large extent if it passes near them; either iron or copper may be used, and existing rain and water pipes, &c., may be utilised; but the joints should be made carefully and tested. Chimneys with soot act as dangerous conductors, and should therefore be protected.

The author does not give any precise directions as to the best form or size of conductors.

LYNILDENS FARLIGHED I NORGE. By H. Mohn, Kristiania. 1875.

The author, having been specially commissioned to enquire into and investigate the danger of lightning in Norway, found that lighthouses, telegraph stations, and other much exposed buildings, which were provided with conductors, did not by far suffer so much as churches, which in the most cases were unprotected.

Out of about 100 churches reported to have been struck by lightning, only three were provided with lightning conductors: on the first, Kongsberg, the conductor was in good order, and the church was comparatively uninjured; the second church, Fossnes, built of wood, had a conductor, but made of zinc wire, which melted, and of course left the church unprotected; on the third, BrÓnÓ (struck 17th October, 1872), the wire had rusted, where it joins the earth, and the church was destroyed.

The author gives a full description of the different cases.

Of 100 churches struck by lightning, fifty-six were totally destroyed, and had to be rebuilt; twenty-four of that number were churches built of stone, twenty-nine of wood; the building material of the remaining three is unknown. It would thus appear that stone buildings are almost as much exposed to be damaged by lightning as wooden ones. Of the above-named churches only one can be said to have been saved by a lightning conductor, viz., Kongsberg. In 1820 the lightning struck the church, set fire to a great part of the wood-work, and did other damage. The tower was then covered with sheet iron. In 1852 the lightning struck the tower again, which, however, then was provided with a conductor consisting of two thin copper plates, 2½ inches wide, fastened on the north and south side of the tower, and both beginning with the iron rod, on which the vane is fastened; but this rod did not end in a point, but in a gilt cross. The conductors were carried down the brickwork of the church to the field, and across the market place, and ended in an old water-butt. When the concussion took place one of the lightning conductors was disabled; but no material injury was done to the tower. In 1872, July 16th, the lightning struck a farmhouse about 700 feet from the above-mentioned church; the farmhouse being about thirty feet, and the tower about 150 feet high.

The construction of a lightning conductor ought to be as follows: It consists of the following three chief parts. (1) The receiver; (2) the conductor; (3) the earth connection. The receiver consists of a copper point 8 inches long and ¾ inch thick; which is screwed into an iron rod, 1½ to 2 inches thick. The screw must fit well and the flats of the copper and iron fittings must be well connected and afterwards soldered round the joint to prevent water and air from rusting the iron. There are various ways of fastening the receiver to the building, but the engineer is generally guided by circumstances. The conductor may be made of iron or copper in the shape of rods or wire twisted like rope. If made of iron rods they should be round and ? to ¾ inch thick; if iron wire-rope is used the thickness must be equal to a rod of ¾ inch; if made of copper the rod must be at least ¼ inch thick, or if made of copper wire-rope ? inch. In both cases the conductor is put in metallic connection with the receiver, and then guided into earth.

The earth connection is merely a continuation of the conductor and must be buried as deep as possible in the earth, and reach the water, if it is to be found.

The end which reaches the water may be constructed in various ways, according to circumstances, but it is of the greatest importance that the earth conductor never gets dry. If there is great difficulty in getting at the water, the earth conductor may be constructed in the following manner. It is made of copper, and has joined to it as many branches as are thought necessary. Each branch has rivetted or soldered to it a copper plate 1 or 2 feet square; they are carried as far away from the building as possible, and buried deep into the earth. Besides this there must be laid an extra conductor, perfectly metallically connected with the chief conductor just under the surface of the earth, alongside of it, out from the building, with as many branches, and as long, as possible. This conductor becomes efficient, as soon as the surface of the earth gets wet through rain, which generally falls during a thunderstorm.

LECTURE DELIVERED BEFORE THE SOCIETY OF ARTS, 28th April, 1875. By R. J. Mann, M.D.

Draws attention in the first place to certain established principles.

Different powers of various substances for conducting electricity.

Electrical induction.

In dull fine weather the surface of the earth negative, the surrounding air commonly positive, the surface of the sea positive.

How a thunder storm begins, gradually approaching cloud, lightning between it and earth. According to Delisle and Petit, a lightning stroke may extend 9 or 10 miles, but for ordinary circumstances the striking distance varies between 650 and 6,500 feet. The lightning stroke follows the line of least resistance, and invariably falls upon the most prominent conducting substance, and passes through substances affording an easy way and offering small resistance without disturbing their molecular condition; shatters bad conductors; heats, sometimes melts, good but insufficient ones.

Describes the various forms of lightning—flash, diffused, sheet, and ball.

A continuous rod of good conducting metal must be carried from the top of the building to the ground. Describes varying carrying capacities of iron, zinc, or copper; recommends from his experience in South Africa, 42–strand rope of 1/16th inch galvanised iron wire.

The disintegrating energy is mainly expended on the extremities of the conductor.

In Natal he used to enclose the top of the rope in a tube of stout zinc, finished at the top by a gilded ball of wood, and he opened the strands of the wire above it into a brush. The French electricians strongly recommend a cluster of points.

The earth contact must be good and damp. The French system of Callaud described.

Gay Lussac recommended that all large metallic masses should be brought into connection with the conductor, and the conductor not insulated from the building. M. Callaud, on the contrary, adopts insulating supports for the conductor, and condemns the connecting of metals in the building.

The metals used in the construction of the buildings may be utilised as conductors; rain pipes, metal ventilating pipes, but not soft metal gas pipes.

Most of those who have given directions for the construction of lightning conductors have paid great attention to the upper and lower extremities of the conductor. They recommend that the upper extremity of the conductor should extend somewhat above the highest part of the building to be protected, and that it should terminate in a sharp point, and that the lower extremity should be carried as far as possible into the conducting strata of the ground, so as to “make” what telegraph engineers call “a good earth.”

The electrical effect of such an arrangement is to tap, as it were, the gathering charge, by facilitating a quiet discharge between the atmospheric accumulation and the earth. The erection of the conductor will cause a somewhat greater number of discharges to occur at the place than would have occurred if it had not been erected, but each of these discharges will be smaller than those which would have occurred without the conductor. It is probable, also, that fewer discharges will occur in the region surrounding the conductor. It appears to me that these arrangements are calculated rather for the benefit of the surrounding country, and for the relief of clouds labouring under an accumulation of electricity, than for the protection of the building on which the conductor is erected.

What we really wish is to prevent the possibility of an electric discharge taking place within a certain region, say, the inside of a gunpowder manufactory.

If this is clearly laid down as our object, the method of securing it is equally clear.

An electric discharge cannot occur between two bodies unless the difference of their potentials is sufficiently great compared with the distance between them. If, therefore, we can keep the potentials of all bodies within a certain region equal or nearly equal, no discharge will take place between them. We may secure this by connecting all these bodies by means of good conductors, such as copper-wire ropes; but it is not necessary to do so; for it may be shown by experiment that if every part of the surface surrounding a certain region is at the same potential, every point within that region must be at the same potential, provided no charged body is placed within the region.

It would therefore be sufficient to surround our powder-mill with a conducting material (to sheathe its roofs, walls, and ground-floor with thick sheet-copper), and then no electrical effect could occur within it on account of any thunderstorm outside.

There would be no need of any earth-connection. We might even place a layer of asphalt between the copper floor and the ground, so as to insulate the building. If the mill were then struck with lightning, it would remain charged for some time, and a person standing on the ground outside and touching the wall might receive a shock; but no electrical effect would be perceived inside, even on the most delicate electrometer. The potential of every thing inside, with respect to the earth, would be suddenly raised or lowered, as the case might be; but electric potential is not a physical condition, but only a mathematical conception, so that no physical effect could be perceived.

It is therefore not necessary to connect large masses of metal, such as engines, tanks, &c., to the walls, if they are entirely within the building.

If, however, any conductor, such as a telegraph wire or a metallic supply-pipe for water or gas, comes into the building from without, the potential of this conductor may be different from that of the building, unless it is connected with the conducting shell of the building. Hence the water or gas supply-pipes, if any enter the building, must be connected to the system of lightning-conductors; and since to connect a telegraph-wire with the conductor would render the telegraph useless, no telegraph from without should be allowed to enter a powder-mill, though there may be electric-bells and other telegraph apparatus entirely within the building.

I have supposed the powder-mill to be entirely sheathed in thick sheet-copper. This, however, is by no means necessary in order to prevent any sensible electric effect taking place within it, supposing it struck by lightning. It is quite sufficient to enclose the building with a network of good conducting substance. For instance, if a copper wire, say No. 4, B.W.G. (0·238 inch in diameter), were carried round the foundation of a house, up each of the corners and gables, and along the ridges, this would probably be a sufficient protection for an ordinary building against any thunderstorm in this climate. The copper wire may be built into the wall to prevent theft, but it should be connected to any outside metal, such as lead or zinc on the roof, and to metal rain-water pipes.

In the case of a powder-mill, it might be advisable to make the network closer by carrying one or two additional wires over the roof and down the walls to the wire at the foundation. If there are water- or gas-pipes which enter the building from without, these must be connected with the system of conducting-wires; but if there are no such metallic connections with distant points, it is not necessary to take any pains to facilitate the escape of the electricity into the earth.

It is desirable, however, to provide for the safety not only of the building itself, but of the system of conductors which protects it. The only parts of this system which are in any danger are the points where the electricity enters and leaves it. If, therefore, the system terminates above in a tall rod with a sharp point, and downwards in an “earth wire,” the external discharge will be almost certain to occur at the ends of these electrodes, and the only possible damage will be the loss of a few particles from their extremities; but even if the rod and wire were destroyed altogether, the building would still be safe.

On Boiler and Factory CHIMNEYS and LIGHTNING Conductors. by R. Wilson. 1877.

The author refers to the wide-spread disbelief in the efficiency of conductors, the common opinion being that metallic bodies, especially when pointed, attract lightning, and are therefore dangerous. This is quite erroneous.

“On an electrified cloud passing over a pointed conductor, the opposite and induced electricity of the earth is discharged from the point of the conductor, and the cloud and air are often thereby neutralized without producing lightning at all. But when a discharge does take place, the conductor offers a line of comparatively small resistance.”

The author further says that, “if electrified clouds be driven to the erection in such masses that the opposite electricity does not stream away from the point of the conductor in sufficient quantities to prevent a spark from passing, the spark or flash will pass from cloud to conductor in preference to any neighbouring point.”

He refers to the safety of conductors, as shown by Sir W. S. Harris’s reports.

When injury to buildings has occurred where lightning rods are fixed, they have been “ignorantly and wrongly applied,” or joints have rusted, the rods been broken, or earth contact has become imperfect.

He refers to Harris and Faraday as to sectional area of conductor. Considers a rope to be better than a rod, as it is less liable to be fractured and to have badly formed joints.

The upper extremity should project into the air as high as the diameter of the chimney top.

The rod should not be inside a chimney, as gases are liable to injure it.

The conductor should communicate with all metal in the chimney.

Insulation is not required.

All contact between copper and iron should be avoided on account of galvanic action.

Earth contact should be tested every year. Anderson’s galvanometer approved of for this.

NOUVEAU PARATONNERRE ACCEPTÉ PAR L’ACADÉMIE DES SCIENCES. Par Jarriant. 8vo. Paris. 1877.

This pamphlet is really a letter by M. Francisque Michel respecting some new patterns of lightning conductors made by M. Jarriant, and submitted to the AcadÉmie des Sciences by M. le Comte du Moncel. The author states that there have been many theories as to the advantage of conductors rising to great heights above buildings, and that, on the other hand, some persons have urged that buildings should bristle all over with points, and thus prevent any disruptive discharge. He thinks that, owing to the translation of the storm-cloud by the wind, these short points will not always have time to act, and says that the only rational plan is to place a conductor high above the house it is intended to protect, and so constructed that it, and it alone, offers a path of scarcely appreciable resistance to the electric discharge. He says that in Germany they put a metal sphere on the top of the conductors, but in France, both the Academy and the Commission of the City of Paris have advised that they should terminate in a point.

M. Francisque Michel says that formerly a conductor was supposed to protect all objects within a cone whose base had a radius of twice the height of the conductor; but that he and M. FÉlix Lucas had investigated the question geometrically, and have arrived at the conclusion that the radius cannot exceed 1·75 of the height. Hence, in many buildings, it became necessary either to increase the number of the conductors or to make them more lofty, both alternatives leading to increased expense. M. Jarriant’s design, which consists of galvanized angle iron bolted together, enables the increased elevation to be obtained at a price twenty per cent. below that of the old patterns. The angle irons themselves offer much surface, their angles are useful for discharging electricity, and they carry at the top the copper terminal recommended by the AcadÉmie.

“The identity of electricity, manifested by friction, with that contained in the atmosphere, was not fully verified until Franklin’s experiment with his kite in June, 1752.”

“In restoring the equilibrium between the opposite electricities of high potential, the discharge will pass by the shortest path, even though a poor conductor, in preference to a longer path through a good conductor.”

The electricity of the earth is usually negative—of the atmosphere, usually positive.

He quotes experiments at Kew to this effect.

The friction of solid and liquid particles against the earth, and against each other in the air, produced by the wind, is a source of atmospheric electricity.

The height of the lower part of the thunder-clouds above the sea in the United States averages about 2,500 feet.

Dense thunder-clouds are good conductors, and are electrified to a certain extent by the induction of the electricity contained in the surface-earth. As electricity accumulates in the thunder-clouds it acts by induction on the surface-earth, and causes a corresponding increase of potential in the earth and the objects thereon.

He alludes to the vitreous tubes (fulgurites), 5 feet to 75 feet deep, as being formed by electricity passing to the subterranean water-bed through sand or other dry earth.

A highly positively electrified cloud within 3,000 feet of a building causes the latter to be intensely negatively electrified by induction.

So also the earth beneath the building and the upper portion of the subterranean water bed.

Whatever offers the least resistance to the stroke will be its chosen path and it will never leave a very good line of conductors, which is in a short path between two opposite electricities, for an inferior one.

151 persons are killed by lightning annually in the United States, France, England, and Switzerland.

He quotes Sir W. S. Harris’s system for the Navy as preventive.

There is no absolute safety anywhere out of doors. It can only be found inside a structure having good conductors, with good earth connections.

Conductors cannot prevent disruptive discharge. They simply furnish a good path for lightning which passes over them without doing any damage.

Protective Area.—A committee appointed in 1875 by the Prefect of the Seine reports as protected, a circular space whose radius is equal to 1·45 [Should be 1·75, see page (67). Ed.] of height of conductor. But this is not always to be relied upon.

It is necessary that a conductor extend along the ridge, gable ends, and eaves of a house, and above each chimney.

Lightning is electricity of very high potential, and the difference of conductivity between the resistance of copper and iron to a lightning discharge is small and practically amounts to nothing.

Iron rod conductors not to be less than 7/16 inch diameter. No case is recorded where such a rod, properly connected with the earth, has been fused or greatly heated by lightning.

Paint or an ordinary amount of rust does not affect conductivity.

A conductor of large surface exercises a much greater protective action than the same quantity of metal in the form of a wire or solid rod.

Not because electricity in motion resides on the surface, but that the expansive action of a discharge may have a wider scope through the metal.

So iron rain water-pipes are good conductors, and should be connected with metal spouting, conductor on ridge, &c.

Cable conductors bend easily and can be made in one length, so often answer better than bars.

If earth connection is good, rusty joints are of little consequence.

Conductors are not to be insulated.

Iron pipes for gas, water, heating, &c., also iron columns extending from basement to near the roof are to be connected with conductor and earth terminals.

The pipes on each side of gas meter are to be connected by iron bands.

Air terminals are to rise about 4 feet above each chimney or other elevated projection.

High steeples to have horizontal conductors round them at every 20 feet in height connected with vertical conductors.

One terminal in the centre of a building not over 25 feet long or wide is sufficient, or one at each end of the ridge. One to each 20 feet of a large building, with one at each end and to each chimney, &c.

When the horizontal portion of a lightning conductor, or path along the roof of a building from ridge to eaves (sic) exceeds 50 feet in length, the path becomes rather indirect for a lightning discharge, which is then apt to select a shorter route through the building.

The upper part of terminal need not be gilt.

Points are practically of no use.

Chimneys are very likely to be struck, owing to the heated air rising from them.

Provide against this by metal caps.

There is danger also, owing to the vapour rising from them, from barns stored with new hay or grain, stables, schools, churches, &c., containing many people, flocks of sheep, &c.

Earth terminals must be in moist ground.

The author quotes Prof. F. Jenkin as to the difference of conductivity between well moistened and perfectly dry earth (as porcelain, &c.) in electricity of low potential, as 1,000,000,000,000 to 1.

Gas and water mains usually 4 feet or so deep in dry earth, therefore not good conductors.

Examples quoted of injury to their joints by lightning, which passed from conductors to the mains.

Suggests, as earth terminal, an iron pipe, 10 feet long, 2 inches diameter, open at each end, perforated at sides, put in vertically, and having the water from pipes for rain and waste led into it.

To be 8 feet from foundation.

Gives engravings of numerous forms proposed for conductors, most of them being defective, and none show improvement on Franklin’s round rod.

Copper rods held by iron staples, and connected with iron earth terminals, are bad, owing to galvanic action.

Copper wires in cable conductors become brittle, and snap when vibrated by the wind; sometimes, also, they are eaten away by electrolytic action.

He gives a drawing of a house protected as suggested by him, viz., by metal rain water-pipes connected with the metal gutters and ridge; also with his improved earth terminal by a good iron bar conductor.

Gas, water, and other pipes are to be connected together, and with conductor.

These often give better path for lightning than the conductors.

But dangerous if without proper earth terminal.

He disagrees with Prof. C. Maxwell’s theory as to disconnecting the metal covering, &c., of buildings from the earth.

Lightning conductors detached from buildings do not afford absolute protection.

Lightning has great affinity for gas-holders, so one of the nearest guide columns should be connected by a metallic conductor with the pipe leading to street main, and also with a vertical earth terminal.

When a telegraph line is altogether metallic, well insulated upon poles, &c., and not metallically connected with the earth, the electricity of a storm-cloud will not exert so strong an inductive influence upon it as upon a line whose ends terminate in the earth.

Line wire is often melted, poles and apparatus shattered, and employÉs sometimes killed.

As a remedy, a galvanised iron wire is now fastened to every fourth pole by iron staples, from 4 inches above the top of the pole to a coil about 10 feet long of iron wire beneath its lower end.

Ueber BLITZABLEITER und BLITZSCHLÄGE in GEBÄUDE welche mit BLITZABLEITERN versehen waren. Von G. Karsten. Kiel. 8vo. 1877.

In this pamphlet Dr. Karsten gives an account of two cases in which buildings that were provided with lightning conductors were damaged by lightning. The author states that the statistics for the year 1873 show that in Schleswig-Holstein twenty-six per cent. of all the cases of fire were caused by lightning; 1/130th part of these cases occurred in the towns and the remainder in the country.

Do lightning conductors guarantee absolute protection? The author answers this question as follows: There is no absolute certainty in empirical matters; each new case may direct our attention to circumstances that had been overlooked. If lightning conductors cannot be said to ensure perfect safety, they certainly afford a very high degree of protection.

The flash of lightning which struck the church at Garding, on the 18th of May, 1877, fractured the conductor in fifteen places and pierced the wall of the steeple in two places. The inefficiency of the conductor resulted from the carelessness with which it was fixed; the line was laid down the north side of the steeple and fastened with twenty-five wall eyes; these wall eyes were hammered too deep into the wall, thus damaging the line and forming a short and sharp bend in each case, besides also unduly straining the wire. The damage to the steeple was the consequence of a neglected secondary circuit. There are an excessively large number of tie-rods in the steeple; the heads of these rods are not connected together, neither are they, except in one case, in close proximity to any of the larger masses of metal that are about the building. The conductor passed close to one of those heads; the south side of the steeple, where the opposite head is, becoming wet through the rain, a secondary circuit was formed, and a return shock followed; the damage to the steeple was trifling.

The rod was provided with a conical point rather blunt but surmounted by a short platinum point. The copper line-wire was of good material—not of a uniform thickness, but at the weakest places not weighing less than 240 grammes per lineal metre (8 oz. per yard or rather less than ¼ inch diameter if solid). The earth-plate was sunk into a well 10 metres deep, and tested faultless after the discharge.

ÉTUDE sur les PARATONNERRES leur CONSTRUCTION leur INSTALLATION. Par Jarriant. 8vo. Paris. 1878.

This pamphlet opens with two pages devoted to the consideration of MichaËlis’s work published in 1783, “De l’effet des pointes placÉes sur le TemplÈ de Salomon;” then it becomes more practical, refers to the Academy of Bordeaux propounding in 1750 the question as to the identity of lightning and electricity, and to Franklin’s letter in the same year to Collinson, giving his reasons for believing in the analogy; states that the experiments suggested by him were repeated by Buffon and Dalibar in March, 1752, and subsequently repeated at Marly before Louis XV. Then the writer refers to the erection of the first conductor in France, to the popular displeasure which it excited, and to the long legal process before the proprietor was allowed to keep it in position.

The author thinks that in many cases it is better to slightly increase the number of conductors than to make them of excessive length, because the latter course causes them to fatigue and jar the roof timbers by their vibration with the wind.

Respecting platinum points he speaks strongly and to the following effect:—“I have already mentioned that Franklin’s first conductor was melted. Since then, the upper terminals of conductors have been made of platinum, because it is the least fusible, the least oxidizable of all metals, and a very suitable one for making into points. Moreover, the sharper a point the greater its preventive action, and hence I condemn every conductor without a platinum point. Although some manufacturers employ simple copper cones, which may certainly last some time without deterioration, believing in the desirability of the points being always in perfect order, I reject their system entirely.”

Few persons are used to making platinum points, it is a Parisian speciality, those which the author prefers, form a cone of about 10 degrees at the opening of the point and are about 1½ inches long, then screwed and soldered into a mass of copper forming a nut on the conical copper rod, which is 1 foot or 1 foot 6 inches long. The platinum point thus mounted can only give rise to a galvanic action so extremely feeble as not in the least to affect the durability of the apparatus. Some persons for the sake of cheapness suppress this platinum point, but they are wrong, the saving is slight and the result defective. The author objects to conductors made of bar iron because the joints are always defective, and if the section be too small they may be so heated as to set fire to the charcoal in which the lower extremity is buried.(!) However, the author prefers a rope, but he does not say whether of iron or copper, and he puts a strand of hemp in the middle so as to make it more pliable.

“Arrived at the ground the conductor ought not to be in immediate contact with the earth, for the damp would slowly destroy it; we avoid this (?) by making it pass through a trough filled with coke. Experience has shown that iron thus buried in coke undergoes no change even during thirty years.... Broken coke is better than charcoal because of the great quantity of water which it absorbs.”

The author then says that after passing through this trough the conductor must be continued into a well, or into very moist earth, and should end with a discharger like a fork with many prongs.

He recommends that all the iron be galvanized.

Although the concluding paragraph, coming from a manufacturer, sounds rather like self-recommendation, it undoubtedly contains important truths. M. Jarriant says:—

“I cannot too strongly advise that in erecting conductors those specialists should be employed, whose studies and constant practice enable them to ensure perfect work. It is necessary also that every workman should remember that in placing a lightning conductor he holds in his hands the lives of men, that he should feel conscientiously interested in the perfection of his work, and, finally, that he should feel that it is a mission which he fulfils, and not a mere matter of trade at which he works.”

This is an interesting report of a careful inspection and an electrical testing, by a skilled electrician, of the lightning conductors at this place. Although most carefully protected by well arranged and adequate copper rods, copper bands, iron rods, and iron tubes, and terminated in points, it was found that the points were covered either with rust or with paint, and that the earth connections were so bad as to render the buildings unsafe, although there was no difficulty in obtaining a good earth at any part of the factory.

ATMOSPHERIC ELECTRICITY. By David Brooks. Philadelphia. 1878. 8vo.

A pamphlet by a distinguished American telegraph engineer, giving his view on the magnitude and origin of atmospheric electricity, which he attributes principally to the friction of air on ice in the Polar regions, and which circulates southwards in the higher regions of the air, and northwards in the crust of the earth. Hence also Aurora Borealis which is always preceded by high winds and most frequent when the earth is covered with snow.

Thunderclouds are usually about 2 miles high and from 13 to 23 miles thick. Lightning is much less frequent in mountainous than in plain countries. Copper lightning conductors are often applied to iron ships and iron buildings, but absurdly, as they are in such cases superfluous.

The author advocates immense earth plates where there are no gas- and water-pipes, which he calls the best lightning rods ever erected, because they are electrically in perfect connection with the earth. The track of a railway makes a capital earth. He has never known an accident where proper conductors were used, whereas he has known many accidents from imperfectly and improperly constructed lightning rods, though of the latest and most approved patents.

CATALOGUE Messrs. A. Collin et Fils, Article PARATONNERRES. Paris. 4to.

The authors state that a Municipal Commission has recommended, to the exclusion of all other points, copper about ¾ inch diameter, terminating in a cone of 30°.

As to the area protected Messrs. Collin refer to the reports of the Academy in 1823 and 1854, admitting, as a limit of protected area, a circumference of which the radius equals double the height of the upper terminal for slightly elevated buildings, and simply the height for towers, &c., but this rule is badly defined.

The authors quote formulÆ based upon the assumed altitude of the storm cloud, but state them to be unreliable.

The Academy in 1854 reports that an electrified cloud is equally attracted at equal distances by a metallic part of the roof and by the terminal of the conductor.

Exposed points of pinnacles, &c., are to be united to main conductors.

If copper be too expensive use iron wire.

The conductors are to be supported at about 10 centimetres (4 ins.) from walls and roofs.

The Academy recommends them to be isolated on glass or porcelain, but the New Commission rejects this, and suggests that all metallic parts be united to the conductor,—also recommends that wells be sunk to water level, as earth connections.

But this would often entail a depth of 20 to 100 metres, or even more. So the conductors may be sunk into moist earth and surrounded with coke, and if necessary, may terminate in a sheet of copper.

A good earth is very important. Connection with water mains advised.

The authors have fixed 8,000 lightning conductors on their principle without failure.

They give engravings of the various parts.

They engrave a diagram of a powder magazine which they propose to protect by a tall isolated lightning conductor fixed at a distance from it, and at such a height as that it will be included in a cone whose radius is equal to the height of the conductor.

THE SCIENTIFIC AMERICAN, NOVEMBER 1st, 1879.

We learn that a lightning rod company in Cincinatti has patented a system of lightning protection, which consists of an iron rod running along the ridge of the building with points at each end projecting upwards. It is supported upon large glass insulators, and has no electrical connection with the building, and no rod running to the ground. It is said that there are many public buildings in Iowa which have been provided with this system of lightning rods.

Professor Macomber, of the Iowa Agricultural College, in reply to an inquiry, says that it would be possible that a house insulated with a glass foundation could be struck by lightning, but adds, “By insulating a building the tendency to be struck by lightning would be very much lessened, and the severity of the shock much decreased. Practical illustrations of this can easily be obtained by means of an electrical machine. A spark can be made to pass from the machine to an insulated body, although the force of the shock will be much less than when not insulated. Practically, it would be almost impossible to insulate a building, because after rain commenced to fall it would wet it so that communication with the earth would be established.”

REMARKS ON THE ATMOSPHERIC ELECTRICITY AND ON THE ACTION OF LIGHTNING CONDUCTORS. By Prof. Dr. G. Karsten. 2nd edition. Kiel, 1879.

The author of this pamphlet, Prof. Dr. G. Karsten, states that thunderstorms are particularly dangerous in Schleswig-Holstein. He attributes that fact to the scarcity of woods in that province, not more than five per cent. of the surface being wooded; whilst in the Prussian empire the proportion of woods is twenty-three per cent.

Woods promote a uniform dampness of the atmosphere and lessen the up-current of air, which up-current contributes considerably to the formation of thunderstorms; and the woods thus cause the discharges of the electricity to take place principally between the clouds.

We do not yet know with certainty what the causes of atmospheric electricity are, but we do know under what conditions or circumstances thunderstorms may occur.

Thunderstorms are only formed when a violent condensation of the rarified particles of water, which the atmosphere contains, takes place. Such a sudden condensation, and the consequent formation of a thunderstorm, may occur when two different masses of air—the one moist and warm, the other dry and cold—intermix rapidly. The former of these currents we call the South, or Equatorial current, the latter the North, or Polar current. If these currents penetrate each other, or intermix slowly, long continued falls of snow and rain ensue; if they mix rapidly thunderstorms are formed during the warmer seasons, and sometimes also during the colder seasons.

The Schleswig-Holstein Provincial Fire Insurance Association alone paid, in sixteen years, the sum of £102,832 (an average of £6,427) for damages caused by lightning. This province loses altogether £12,500 per annum through fires caused by lightning.

The author’s very interesting remarks on the construction of lightning conductors are briefly summarised in the following general rules:

1. Copper and iron form the best materials for lightning conductors; lead and zinc may be used for secondary conductors. (Nebenleitungen.)

2. If the conductor be constructed of iron, it should weigh from 1,200 to 3,400 grammes per metre (2½ lbs. to 7 lbs. per yard), according to its length; a copper conductor should weigh, under the same circumstances, from 250 to 600 grammes per metre (½ lb. to 1¼ lbs. per yard).

3. The conductor must be connected with all the projecting corners and pointed parts of the building.

4. There must be no sharp curves or bends in the conductor.

5. The conductor must be connected with all the large and extensive masses of metal that may be about the building. This connection may be made by wires leading towards the rod, as well as in the direction of the earth contact.

6. The rods must be surmounted by good points, which must not be liable to be fused by the discharges of the electricity.

7. The height of the rods must be in proportion to the size and shape of the buildings; but it is better to erect several short rods than one extraordinarily long one.

8. In making the connection with the earth all sharp curves must be avoided.

9. The underground part of the conductor must be made of galvanized metal, so as to minimise the effects of oxidation, or, in case a layer of coke is used, to prevent the action of the sulphur.

10. The earth-contact should terminate in a plate, which, if possible, should always be immersed in water. If this can be so arranged the plate must have a surface ?th of a square metre (1 foot square) for conductors for small buildings, whilst a plate of a surface of 2 square metres (5 feet square) will be sufficient for conductors for the largest buildings.

11. Where a permanent contact with water cannot be established, several plates of a larger size must be used, and laid in a stratum of coke.

12. In the case of very large buildings, provided with several rods and secondary conductors, several earth-contacts should be made which should be connected with each other.

With reference to the upper terminal point, the author remarks, in an appendix to the second edition of his pamphlet, that it should be made of a conical form of a basis of from 20 to 30 millimetres (0·8 in. to 1·2 in.), and of a length of 150 millimetres (6 inches); it must consist of pure copper and be gilded. It is useful to provide it with a platinum needle 15 millimetres (half an inch) long, and about 4 millimetres (0·2 inch) thick at its base; or with a cone of chemically pure silver, the proportion between whose base and height must be as 2 : 3.

LIGHTNING CONDUCTORS. By Richard Anderson, London, 1879.

The following are brief references to some of the principal facts recorded in this volume:—

1600 A.D. Dr. Gilbert showed that magnetic and electrical phenomena were emanations of one force.

1650. Otto Von Guericke constructed a little electrical machine (mainly of a ball of sulphur on a revolving axis).

Sir I. Newton constructed a machine of glass, but used it merely for amusement.

1675. The polarity of a ship’s compass was found to be reversed by a stroke of lightning.

1708. Dr. Wall said that the light and crackling of rubbed amber seemed in some degree to resemble lightning and thunder.

1709. F. Hauksbee, F.R.S., showed the similarity between the electric flash and lightning.

1720. S. Gray, F.R.S., showed this by experiment, but was discredited.

1745. The first great step in this science was made at Leyden, by J. N. Allamand and P. van Musschenbroek, who discovered the properties of the Leyden jar. The priority of this invention disputed by Dr. Winckler, of Leipzig; a mania for experiments arose. Louis XV. tried them, unsuccessfully, on 180 of his Guards; but with perfect success on 700 Carthusian Monks.

1746. Dr. Franklin, of Philadelphia, saw some electrical experiments, and in

1747 received a glass tube and some books on electricity from London; then began to make experiments; sold his business, bought apparatus and made electricity his study. Discovered that electricity passed most easily and quickly through sharply pointed metals; that it was positive and negative; and that lightning and electricity were identical. He sent these results to the Royal Society, who refused to allow them to appear in their Transactions; he then published them in a pamphlet. It was not appreciated in England, but met with great applause in France, and was also translated into German, Italian and Latin.

1747. The subject was taken up in England in a thoroughly practical manner. Dr. Watson, Mr. Folkes, Lord C. Cavendish, Dr. Bevis, &c., experimented on a wire stretched across the Thames. The charge was found to come back by the water. The same result followed through moist earth. A gun was fired at a distance of four miles; the passage of the charge appearing to be instantaneous.

New experiments were made by Dr. Watson with glass rods, 2 and 3 feet long and 1 inch diameter. These showed that the rods, &c., contained electricity only as a sponge holds water.

1752. Experiments by Messrs. Dalibard and De Lor, at Marly-la-Ville, near Paris, in May, described.

1752. July. Franklin tried his Kite successfully, then his fame was established, and he erected, on his own house, the first lightning rod.

1753. Prof. Richmann, St. Petersburg, was killed whilst experimenting. The use of conductors opposed, violently in France, by AbbÉ Nollet.

1755. An earthquake at Massachusets, was laid to the charge of the numerous lightning conductors. Franklin pushed their use by means of his publication, “Poor Richard,” which had an enormous circulation; particulars given showing success of lightning conductors.

1762. The first lightning conductor used in England, and Dr. Watson asked to send in designs for lightning rods for ships. He did so, but in an unpractical way, and they were disused.

1764. St. Bride’s steeple struck.

1769. The Dean and Chapter asked Royal Society for advice as to protecting St. Paul’s. Committee of Royal Society disagreed as to whether rods should be pointed. Pointed rods were used.

1769. The first conductor fixed to a public building in Europe was to a church steeple in Hamburg.

De Saussure, at Geneva, had some difficulty in explaining to the citizens that his conductors were not dangerous to his neighbours. There was a great fear, generally, as to their use, e.g., a lightning rod was erected, secretly, by the Priests at the Cathedral of Siena, and excited great terror in the townsmen when discovered, but a terrific stroke of lightning left the tower uninjured.

1772. Dr. Ingenhousz’s experiments.

1774. The University of Padua protected by conductors.

1777. A building at Purfleet was struck though it had a conductor, but this was shown to be defective.

Sir J. Pringle had to resign his Presidency of the Royal Society because he advocated points, but experiments were made and ended in favour of points.

1778. The Venetians decreed that lightning rods should be erected throughout the Republic.

1819. Electro-magnetism discovered by Œrsted.

1822. Sir W. S. Harris took up the question of providing good conductors for ships, and afterwards made a list of 250 accidents to ships in 40 years; also of 200 seamen killed or wounded in that time. At this time no importance was attached to the subject in England, except in the case of Sir W. S. Harris. He insisted on the necessity of lightning rods. A commission of inquiry was appointed by H.M. Government to investigate the best method of applying lightning rods to H.M.’s ships, and they reported (in 80 pages folio) that lightning rods were rather new fangled things, but might be tried, without special harm to anybody. So most ships were fitted with them after Sir W. S. Harris’s design. He was knighted in 1847. An iron built ship, metal rigged, is as well protected from lightning as Solomon’s Temple. Harris combated the opinion of those who said that lightning rods attracted lightning.

Even in 1826 a government engineer recommended, on this ground, that all lightning rods should be pulled down, and, in 1838, the Governor-General of India ordered this by the advice of his “scientific officers.” This was not countermanded until several buildings had been destroyed.

Army circulars are now regularly issued, containing Sir W. S. Harris’s suggestions. (These quoted by Mr. Anderson).

Sir C. Barry suggested that Sir W. S. Harris should design lightning conductors for new Houses of Parliament. He reported in 1855. He used conductors of 2 inch copper tubes, ?th-inch thick, to towers and other elevated parts, secured to masonry by metal staples. The cost was £2,314.

As to conductors, Le Roy recommended that they should rise not less than 15 feet above chimney and summit of any edifice.

Mr. Anderson gives technical names of parts of lightning rods in different countries. Chains first used, and gave rise to many accidents. Tin and lead conductors tried; lead especially, from its easy application to sharp curves, &c., but it is liable to be broken, and is a bad conductor; so it went out of use.

Some particular buildings are constantly under attack from lightning, e.g., Church of Rosenberg in Carinthia, not standing in a very high position, but greviously damaged in 1730, &c.; rebuilt in 1778, with lightning rod, and not injured since. Some of these effects may be explained on meteorological grounds: the height and thickness of the charged clouds only slightly varying, perhaps, in districts where there are prevailing winds. The height of clouds sometimes enormous. Instances are given of their being 15,000 to 25,000 feet above the sea. But sometimes clouds are almost flat on the earth, two instances are given of this. A remarkable and often fatal discharge is the “return stroke,” always less violent than the direct stroke, but often very powerful, and caused by the inductive action exerted by a thunder-cloud. Men and animals are charged with opposite electricity to the cloud. When the latter is discharged by the recombination of its electricity with that of the ground, the induction ceases, and all bodies charged by induction return to a neutral condition. Hence the dangerous “return stroke.” Lord Mahon first demonstrated this by experiment. As to origin of atmospheric electricity, De Saussure considered it due to the evaporation of water by the sun’s heat. Peltier (1765–1845) considered the earth itself to be one immense reservoir of electricity. As light comes from the sun, so electricity is generated by heat from the interior of the globe. No electricity is produced by atmosphere, nor held by it, except temporarily.

There is no recorded case in which a well made lightning rod, with “good earth,” did not do its duty.

In 1822 there was an extraordinary number of thunder storms in France, so lightning rods were ordered by Minister of Interior for all public buildings, and he applied to the Academy of Sciences for advice. 1823. A Committee (Gray Lussac, &c.) reported. They laid it down, as a rule, that a lightning rod protected a circular area, having a radius of double the height of the rod; and they said nothing about regular inspection of lightning rods. So disasters occurred, and another Committee was appointed (Pouillet, &c.). They reported 1854. The theory as to the protected area was abandoned. It was recommended that lightning rods should have as few joints as possible. The joints to be well soldered, the points to be of copper (not platinum), and not to be very finely pointed. The rods to be of copper, not iron. The Louvre was well protected by lightning rods, but slightly injured, 1854. Another Committee was appointed, and, 1855, Pouillet again reported on its behalf. It recommended that the points (always of copper) should be thicker, and the rod to have a never-failing connection with water or moist earth, 1866. Several French powder magazines were struck though provided with lightning rods, and the Minister of War asked the Academy for another report. Another Committee (Becquerel, &c.) was appointed, and, 1867, Pouillet again reported. He defines lightning as an immense electric spark passing from one cloud to another, or from cloud to earth, to restore equilibrium. The best protection for a building would be iron rods surrounding it on all sides, and passing deep into ground. Conductors should be inspected every year.

The conductor now remains essentially as Franklin invented it. Of the inner nature of “lightning” we are utterly ignorant. The first conductors were always of iron as being cheap.

Sir H. Davy pointed out the different conducting powers of different metals. Becquerel, Lenz, Ohm, and Pouillet made similar investigations, with the following results:—

	Silver.	Copper.	Lead.	Tin.	Iron.	Iron = 1 Copper =
Davy	109·1	100	69·1		14·6	6·85
Becquerel	73·5	100	8·3	15·5	15·8	6·33
Lenz	136·25	100	14·62	30·84	17·74	5·64
Ohm	35·60	100	9·7	16·8	17·4	5·75
Pouillet	81·26	100			18·2 to 15·6	5·49 to 6·41

1815. Brass wire rope generally used in Bavaria, but a steeple was struck down though with a brass wire conductor 1 inch diameter. The real defect was “bad earth,” but attributed to bad form of conductor; so this was abandoned. Brass not a reliable metal, and often destroyed by smoke. Purity of copper essential.

Professor Matthiessens’ experiments shewed that the conductivity of copper varied from—

	Pure	100·
to	Australian	88·86
	Russian	59·34
and	Spanish, Rio Tinto	14·24

Hotel de Ville, Brussels, lightning rods designed by Professor Melsens on the principle of a great number of small ones in preference to one of large size, and covering building with network of metal, having many points and many earth contacts. He considers that the relation of section to surface of the lightning rod has a marked and definite, though unknown, result.

Author describes weathercocks and methods of fixing them.

Lightning rods generally—methods used in France: Terminal rods, usually of wrought iron, galvanized; their height depends on the size and area of building it protects. This is generally to be considered to be within a cone of revolution, of which the radius = height of rod above ridge × 1·75.

Points described. The conductors are of iron, rebated, soldered, and bolted at joints, with lead between. Bent plates of copper introduced to provide against contraction and expansion. In large buildings, metallic connections are formed on ridge by iron bars-¾ in. × ¾ in.

Precautions are taken against the destruction of iron underground, viz., by enclosing it in vertical sprints of wood, tarred or creosoted, rising a few inches above ground, or by a coating of tar or by a wrapper of sheet lead. The earth connection is a trough filled with broken charcoal, through which the conductor passes, ending in several branches, or in a grating between layers of charcoal. Galvanized iron cables sometimes used, and (rarely) copper of ½ in. diameter.

America. Gutters and water pipes, &c., used where possible. If the roof be of wood, slate, &c., a conductor is laid along ridge, and connected with gutters and rain water pipes. If these latter be less than 3 in. diameter, the conductor is often extended from roof down the side of building close to the pipe. All metal chimney caps, railings, water and gas pipes, and other large or long pieces of metal, inside and out, are connected with conductor. The upper terminal usually projects 4 ft. above chimney or other highest part of building. It is a round rod, 7/16th in. diameter, hammered out to join it to conductor. A building 25 ft. wide and broad has one terminal in centre and one at each end. In larger buildings, one terminal to each 20 ft. of roof. Not always pointed.

Steeples have horizontal conductors at every 20 feet, connected with vertical conductors, to provide against discharge in centre, caused by deflection of discharge in the air by rain. Conductors are fixed to buildings by iron staples or straps; the earth connections are similar to ours. Also are used iron pipes, about 3 in. diameter and 10 ft. long, placed vertically in moist earth and carefully connected with conductor.

Newall’s system: Copper conductors are the best, and in the end, cheapest. Terminal rods are usually 3 to 5 ft. long, and ?th to ¾ in. diameter, branching out at top.

German “reception rod” described as being of iron, 10 to 30 ft. long; the area of protection theory discredited. The electric fire, seeking its nearest path to earth, is not to be diverged from it to the rod. These high rods of no use except, e.g., near high trees, and are often dangerous from being blown down. Barns containing new hay are likely to be struck, as hay sends out stream of warm air.

Designs explained for protecting private houses by short terminal points to chimneys, gables, &c. A copper rope at least ?th in. diameter should be used; a copper rod, ½ in. diameter, has never been fused, so far as is known. In chimneys of manufactories, where rope is liable to corrosion, a greater thickness should be used.

Laughton-en-le-Morthen steeple injured, though with lightning rod, but this was only a small, thin copper tube, ?th in. external diameter, and 1/32 in. thick; weighing 8oz. per foot, or equal to a rod about 0·12 in. diameter, the joints were corroded, and the earth contact was imperfect. Nevertheless, only one buttress was injured. It is of little consequence whether the conductor be inside or outside, if it be carried to earth by the shortest route. At first it was more generally put inside in France, but this was given up for fear of accidents. But it is beyond controversy that a good conductor is absolutely harmless to all surrounding objects, and a man might lean against a copper half-inch rod, carrying off a heavy stroke of lightning into “good earth,” without being aware of its passing.

It is useless and dangerous to isolate conductors from buildings. All masses of metal should be connected with conductors.

Prof. Clerk Maxwell’s theory described (as to disconnecting the conductors, &c., from the earth): He states that it is not necessary to connect masses of metal, as engine tanks, &c., if entirely within the building, unless a conductor as, e.g., telegraph wire, water or gas pipe come into the building from outside, then they must be connected with conductor.

List of accidents from lightning, also deaths or injuries in England and Wales, Prussia, United States, Sweden and Austria.

Particulars of damage to St. George’s Church, Leicester, 1846, and to West-end Church, Southampton. Also, to Merton College, Oxford, and St. Bride’s Church, Fleet Street, none of these having lightning rods.

Wrexham Church struck, this had a copper conductor, but it was too small and the earth contact was doubtful.

List of buildings struck at home and abroad from 1589 to September, 1879, the authorities for the statements being given.

List of powder magazines struck between 1732 and 1878.

Earth Connections. Franklin’s report, 1772, strongly urges the importance of this, in speaking of the powder magazine at Purfleet. In ordinary cases, moist earth is sufficient, but in such a case as this he recommends that a well should be dug at each end of magazine, with 3 to 4 ft. of water in it.

The importance of “good earth” is shewn by numerous accidents to buildings, as, e.g., in 1779, the church of St. Mary, Genoa, and, in 1872, the cathedral of Alatri, in which latter case, the discharge left moist earth to pass off by a water pipe, which it broke; but the church was uninjured. Also at Clevedon Church, where the conductor passed into a drain which was dry, but the stroke merely injured one buttress and passed off by gas and water pipes.

Mr. Anderson states that earth contacts must be large. That it is important that metal work be connected with lightning rod in at least two parts, to realize a closed metallic circuit, and so offer entry and exit. The earth contacts of the eight conductors of the Hotel de Ville, Brussels, described, viz., their being enclosed in an iron box, 8 in. × 3 in. × 3½ in., with three series of conductors (details given): one passing into a well, another to the gas main, the third to water main.

In ordinary buildings, the grating, with charcoal, coke, or cinders, &c., as before described, may be sufficient; but with large buildings, contact with water is absolutely necessary.

Periodical inspection. Author strongly urges this because conductors deteriorate from action of wind and weather above ground; the “earth” often becomes bad, owing to new drains, &c.; buildings may be altered in regard of the quantity and position of metals. An instance is given of damage to a building owing to the change of position of iron safe. Conductors are often displaced by workmen; and the number and position of new gas and water mains, new trees, &c., also influence the power of conductors.

Appendix. This contains a very full list of books relating to lightning conductors.

REPORT upon LIGHTNING DISCHARGES in the Province of Schleswig-Holstein. By Dr. Leonhard Weber. 1880. 8vo.

The serious damage caused in Schleswig-Holstein by lightning led to an official inquiry into the subject, the following is an abstract of the first report of the commission.

It is stated that trees, by their gradual but uninterrupted discharge of electricity, have a dispersing effect upon thunder-clouds, and tend to lessen the energy of lightning. In six cases out of the twelve examined, houses with trees close by, were struck, but not so heavily as in another case where the building had no protection whatever. Trees do not, however, afford complete protection to neighbouring buildings, their conductive capacities not being sufficient to convey, in the immeasurably short time required, such heavy discharges of electricity as lightning flashes. This is instanced by their being often wholly, or partially, destroyed by the current, or, as occurred in four cases, by their passing it over to better conductors, buildings, &c.

If a thunder-cloud passed over a perfectly plane surface, the discharge would take place in a vertical line between earth and cloud, but prominent objects, such as isolated trees, buildings, lightning conductors, and iron pumps, reaching down to underground water, act as attractive points, and divert the discharge, the path of which is also influenced by any conductors which happen to come between them and the thunder-cloud, such influence depending upon the capacity of the conductors. So that, generally an electric discharge chooses that path which, taking the distance into account, offers the best means of conduction.

It is frequently found that inflammable material is struck by lightning without being ignited, on account, it is presumed, of the short duration of discharge not allowing the material to become sufficiently hot to burn, but whether the duration of discharge is dependent upon the nature of the charge of the thunder-cloud, or solely upon the condition of the objects struck, has not been ascertained. The latter is, however, not without influence, as in two of the four cases which resulted in fire, the cause was presumably due to newly gathered hay stored at the top of the houses struck, and in the other two cases to trees, which were struck at the same time, the hay and the trees being bad conductors, and prolonging the duration of discharge.

Four cases are given of buildings having lightning conductors being struck.

The first case is that of a windmill, the conductor of which terminated in a sheet of metal placed in a well near the building. The discharge was exceedingly heavy, but beyond the platinum point being almost entirely fused, no other damage was done.

The second is that of a house with two separate lightning conductors, each ending in a copper plate, spirally coiled up, and laid in underground water. One of the conductors was struck, and the lightning passed from it, and, running horizontally along the thatched roof of the house, descended by the other, causing no damage.

The third case refers to a church and, adjoining it, a school building. A portion of the discharge was diverted from the conductor by an anchor in the church wall three metres off (which it magnetized), and forced its way through the ceiling of the school-house to a number of gas brackets, which were turned up towards the ceiling. It was ascertained that the ground floor of the house was completely under water, and well connected to earth through the gas mains and an iron pump, a good continuous conductor thus being formed.

Accordingly, the report recommends that lightning conductors should be connected to the large masses of metal, such as gas and water mains, which are found in our houses.

In the fourth instance a church had a lightning conductor, which was connected to the top of two large iron supports running through the steeple to the nave, and which terminated in a coiled earth-plate, 1 sq. metre (11 sq. ft.), supposed to lie in water 7 metres (23 ft.) underground. The lightning struck the conductor and, passing to the iron supports, sprang from one through the outer wall, close to an iron window frame, and from the other across the stucco ceiling, going to earth 100 feet off through the altar gilding, which it blackened. It was subsequently found that the copper earth-plate was only ? metre (1 ft. 1 in. sq.), and that it was buried loosely round the rod in dry sand, the rod itself reaching 2 to 3 metres further down, and just touching water without an earth-plate, and also that the two supports had no earth connection, thus forming a great danger instead of a safeguard to the church.

DIE KONSTRUKTION und ANLEGUNG DER BLITZABLEITER zum Schutze aller Arten VON GEBÄUDEN SEESCHIFFEN und TELEGRAFEN STATIONEN. Von Dr. Otto Buchner. Weimar. 1867. 8vo.

The book is divided into two parts:

1. General, or Introductory, and

2. Practical.

The first, or Introductory part, is sub-divided into:

1. Historical and statistical notes;

2. The theory of atmospheric electricity, and of the lightning conductor; and

3. A chapter on natural lightning conductors.

The great philosopher, Lichtenberg, of Gottingen, said in the year 1794: “People are struck and their dwellings are destroyed by lightning because they will have it so. It does not matter to us whether parsimony, carelessness, ignorance, or anything else is the cause of this.” The author asserts that this dictum may be equally applied to the present generation.

Professor J. H. Winkler, of Leipzig, discovered, in the year 1746, that electricity is the principal cause of thunderstorms.

The first lightning conductor in Germany was erected 1769, at Hamburg, on the steeple of the Jacobi Church.

Between the years 1835 and 1863, a period of 19 years, 2238 persons were killed by lightning in France. The maximum in one year (1835) was 111 and the minimum 48. The total number of persons struck by lightning amounted to 6714; of this large number 1700 persons would have escaped, if they had been careful to avoid the neighbourhood of trees, whilst the storms were raging. The greatest number of the accidents caused by lightning occur during the months of July and August; not a single fatal case is on record for the months of November, December, January, and February. The annual average number of persons killed by lightning was 3 in Belgium, 22 in England, and 10 in Sweden. In the low-lying Departments of France the average is 2 or 3; the average increases rapidly for the Mountainous Departments to 24, 28, 38, 44, and (in Auvergne) 48. The per centage of males in France is 67, females 10, and in the remaining cases the sex was not stated. In Prussia the proportion is 184 males to 105 females, in Sweden 5 males to 3 females.

The largest number of persons killed by one discharge is 8 or 9.

The author states that the return shock is only mechanical in its effects.

Professor MÜller lays down the following conditions for lightning conductors:—

1. The rod must end in a very sharp point.

2. There must be no want of continuity between the extreme point and the earth contact: and

3. The different parts of the conductor must be of the requisite dimensions.

In practice we find that the first mentioned condition is incorrect, as sharp points are too liable to be fused.

The rod must be made of a pyramidal or a conical form. Short rods of not above 2 metres (6 feet 7 inches) in length may be made of a cylindrical form. The best form of rod is one tapering from a base of from 50 to 60 millimetres (2 inches to 2·4 inches) in diameter to a diameter of not less than 14 millimetres (0·56 inches). As it is difficult to fix rods of a height of 10 metres (33 feet), it is better to erect one long rod, and several shorter ones on different parts of the roof and connect them together. The principal rod should have a height of from 2½ to 3 metres (8 to 10 feet) and the secondary rods (Nebenstangen) should be at least 1 metre (3 feet 3 inches) high.

The form of point universally used in Germany is a strongly firegilded copper cone.

Kuhn advocates the use of chemically pure silver for the points. His arguments in favour of this metal are incontrovertible. The conducting power of silver is 1·36; that of pure copper being 1. The fusibility of silver (1,000 c.) is sufficiently high for the purpose. The atmosphere, unless it contains sulphur in a gaseous or a liquid form, has no effect on silver. Silver is cheaper than platinum, and not more expensive than a gilded copper cone, and it can be easily soldered to other metals.

The point should be screwed on, as well as soldered to the rod. All other but the conical form of point should be rejected.

The best material for the earth contact is galvanised iron.

As regards the protection of sea-going vessels, Snow Harris’s arrangement, converting, as it were, the vessel into one mass of metal, is perfect.

The first practicable lightning conductor for the protection of telegraph wires was constructed by Steinheil in 1846. His arrangement was somewhat modified by Breguet and Fardely. Meiszner introduced a real improvement.

On the Prussian railway telegraphs two “point-systems” are in use, one for small stations, and the other for larger stations.

It is desirable that all lightning conductors be examined once a year. The metallic connection throughout must be perfect, the point must be kept free from rust, and the earth contact must be good. The whole circuit should also be tested by means of a battery and a galvanometer.

EARTH CONNECTIONS OF LIGHTNING CONDUCTORS. By Lieut.-Col. Stotherd, R.E.

Arguing from the case of a powder magazine at East London, Cape of Good Hope, when the iron conductor was led into a cemented water-tank, frequently dry, and where it was destroyed, the author raises two questions:

1. Should such tanks be used for earth?

2. Is iron the proper metal to use?

He gives a decided negative reply to the first, and advocates the use of galvanized iron properly protected from atmospheric action. He suggests rods 1 inch in diameter, or bands 2in. × ?in. thick.

In the discussion which followed it was mentioned that the ground about Torquay is so insulated that plates had to be carried out to sea to secure a good earth for the telegraph there, and that of the numerous churches which had been inspected, there was not a single conductor that could be passed. It was pointed out that when copper conductors were fixed with iron wall-eyes—a frequent thing—galvanic currents were set up, and the conductor destroyed at the ground line.

It was stated that the earth connection of a supposed perfect conductor was found to be equal to a resistance of 1,000 Ohms.

Mr. Preece, Major Malcolm, R.E., Dr. Mann, Mr. Pidgeon, Mr. Kempe, Mr. Graves, Mr. Spagnoletti, and Mr. Latimer Clark, took part in the discussion.

REMARKS on some PRACTICAL points connected with the construction of LIGHTNING CONDUCTORS. By R. J. Mann, M.D., F.R.A.S. (Quarterly Journal Meteor. Soc., October, 1875).

States that there are certain principles accepted as established facts, e.g., that conductors should be of metal of high conductivity, and of adequate dimensions. That in 1854 the French electricians held that a “quadrangular iron bar ¾ in. diameter, was sufficient in conducting power for all purposes.” Since then, wire ropes, owing to their pliability, have nearly superseded solid rods, and copper has been preferred to iron because of its higher conducting power and less liability to oxidise. But provided that the iron be galvanized, and of five times the sectional area of a copper conductor, considers the metal immaterial.

Author states that the resistance of a conductor increases with its length, therefore sectional area of conductor must be increased for lofty buildings. Modern French electricians employ copper rope 0·4 to 0·8 in. diameter. M. R. Francisque Michel considers galvanized iron wire rope 0·8 in. diameter sufficient for all ordinary cases. Copper wire rope 0·5 in. diameter (6¾ oz. per foot) recently applied to St. Paul’s Cathedral.

Importance of perfect earth connection strongly insisted upon, but it is matter of some difficulty, and the oxidation of the earth terminals, and their inefficiency doubtless lead to most of the reported failures of lightning conductors. Author quotes Pouillet and Becquerel, as saying, that for the efficient discharge of the lightning, which could be carried by a copper rod 0·8 in. diameter, contact must be obtained with 1,200 square yards of moist earth, but this large requirement can only easily be obtained in towns by connection with the water mains. Various modes of obtaining adequate earth contact by iron harrows, Callaud’s grapnel in basket of coke, &c., described.

Explains the rationale of testing goodness of earth currents by the galvanometer. Calls attention to the destruction of upper terminals of conductors to factory chimneys by the emission of sulphurous fumes, and suggests that they might be cased in lead.

Calls attention to the importance of every joint being made absolutely perfect.

Urges the superiority of points for upper terminals, owing to their facilitating silent discharge, and rendering lateral discharges from the conductor less probable.

Thinks that multiple points of copper kept fairly sharp and clean are, on the whole, the best upper terminals.

Considers that all large masses of metal in a building should be connected with the conductor; but quotes M. Callaud, who holds the opposite view. Dr. Mann, however, points out that if the conductor be efficient and perfect, the accidents which M. Callaud contemplates, and on which he bases his arguments, could not occur.

Calls attention to the ready path afforded by the column of heated smoke discharged by chimneys, and hence alludes to the placing of a coronal conductor, as well as a multiple point on important chimneys.

Suggests the utilization of rain water pipes, by perfecting their joints, and securing a good earth connection at their base.

ON THE PROTECTION OF BUILDINGS FROM LIGHTNING. By R. S. Brough, 4to, Mussoorie, 1878.

A carefully prepared theoretical and practical paper, adapted for use in India. Author advocates the use of iron from its higher temperature of fusion, and greater specific heat than copper, its long protection from decay by galvanization and its cheapness. He prefers wire cables from the absence of joints in them. He gives precise instructions for the formation of a good earth, and advocates periodic electrical tests.

LIGHTNING CONDUCTORS. By Professors Ayrton and Perry. (Journal Society of Telegraph Engineers. Vol. V., 1876, p. 412.)

The authors controvert Clark Maxwell’s views that a building would be perfectly protected from lightning by being enclosed in a network, or cage of wires, without the use of the earth. They object to the application of the laws of static electricity alone to such a case. Current induction intervenes, and this is not subject to the screening action of a cage. Hence, though a metallic cage may assist the protection of a house, it does not do so perfectly.

ON THE PROPER FORM OF LIGHTNING CONDUCTORS. By W. H. Preece, C.E. (British Association Report, 1880).

Author states that ever since lightning conductors have been used, there have been disputes as to whether the discharge passes over the surface of conductors or through their mass. Snow Harris, Henry, Melsens, and Guillemin have held that it passed over the surface; Faraday held the opposite view.

The arguments in favour of the surface form are, in the opinion of the author, deductions from exploded theories, from imperfect experiments, or from erroneous interpretations of well ascertained facts. No direct experiments have ever been made to solve the question, as far as the author knows. Quantities of electricity, that is static discharges from condensers, are in incessant use for telegraphic purposes, and are found to follow exactly Ohm’s laws, even with the most delicate apparatus. The knowledge of the flow of electricity through conductors, of the retarding influence of electrostatic capacity upon this flow, and of the distribution of charge, has become so much greater of late years through the great extension of submarine telegraphy and the labours of Sir William Thomson, Clerk Maxwell, and others, that the author questions if any English electrician would now be found to argue in favour of the surface form. Nevertheless, as ribbons and tubes still continue to be used, and it appeared very desirable to settle the question experimentally, the author determined to try and do so.

First Experiments, June 28, 1880.

Dr. Warren de la Rue, who is always ready to place his splendidly equipped laboratory at the service of science, not only allowed the author to use his enormous battery and his various appliances, but aided him by his advice, and assisted him in conducting the experiments.

Copper conductors, 30 feet long, of precisely the same mass, (a) drawn into a solid cylinder, (b) made into a thin tube, and (c) rolled into a thin ribbon, were first of all obtained. The source of electricity was 3,240 chloride of silver cells. The charge was accumulated in a condenser of a capacity of 42·8 microfarads. It was discharged through platinum wire of ·0125 diameter, of different lengths. The sudden discharge of such a large quantity of electricity as that contained by 42·8 mf. raised to a potential of 3,317^[5] volts is very difficult to measure. It partakes very much of the character of lightning. In fact, the difference of potential per unit length of air is probably greater than that of ordinary lightning itself. It completely deflagrates 2½ inches of the platinum wire, but by increasing the length of the wire it could be made to reproduce all the different phases of heat which are indicated by the various shades of red until we reach white heat, fusion, and deflagration. Hence the character of the deflagration, which is (by its scattered particles) faithfully recorded on a white card to which the wire is attached, is a fairly approximate measure of the charge that has passed, while the length of wire, raised to a dull red heat, is a better one, for any variation in the strength of the current within moderate limits is faithfully recorded by the change of colour.

Experiment 1.—Similar charges were passed through the ribbon, tube, and wire, and in each case 2½ inches of wire were deflagrated. No difference whatever could be detected in the character of the deflagration.

Experiment 2.—Ten inches of wire were taken and similar charges passed through. In each case the wire was raised to very bright redness, bordering on the fusing point, and in two cases the wire broke. In each case the wire knuckled up into wrinkles, and gave evidence of powerful mechanical disturbance. The same wire was not used a second time. No difference could be detected in the effect through the different conductors.

Experiment 3.—Silver wire of the same diameter and length was used, and similar charges transmitted through it. Redness was barely visible, but the behaviour of the wire was similar in each case.

The conclusion arrived at unhesitatingly was, that change of form produced no difference whatever in the character of the discharge, and that it depended simply on mass.

Second Experiments, July 19, 1880.

As it might be urged that the length of conductor tested was so short, and its resistance so small that considerable variations might occur and yet be invisible, similar lengths (30 feet) of lead—a very bad conductor, its resistance being twelve times that of copper—were obtained, drawn as a wire, made as a tube, and rolled as a ribbon, each being of similar weight.

Experiment 4.—Charges from the same condenser, 42·8 mf., but with 3,280 cells, were passed through, and the discharges observed on 6 inches of platinum wire 0·0125 inch diameter, which in each case was heated to bright redness. No variation whatever could be detected, whether the wire, the tube, or the ribbon were used.

Experiment 5.—In order to form some idea as to how closely any variation in the character of the discharge could be estimated, a long piece of platinum wire was used, and the length adjusted until just visible redness was obtained; then a diminution of 10 per cent. (3 feet) produced a marked change to dull redness, and further excisions raised the temperature to brighter and still brighter red.

The conclusion arrived at was that any change in resistance of 5 per cent. would have been clearly and easily discernible.

It therefore appears proved that the discharges of electricity of high potentials obey the laws of Ohm, and are not affected by change of form. Hence, extent of surface does not favour lightning discharges. No more efficient lightning conductor than a cylindrical rod or a wire rope can therefore be devised.

ÉTABLISSEMENT DE LA FORMULE RELATIVE AU RAYON D’ACTION DES PARATONNERRES. Par Emile Lacoine. (L’ElectricitÉ, October, 1880.)

This author gives a formula for determining the area protected, which he considers to vary with the height of the storm cloud, and the elevation of the ground. He states that the mean elevation of the storm clouds at Constantinople is as low as about 325 feet. He says that conductors placed near the extremities of a building have their radius of protection diminished, and therefore recommends a line conductor running round the building. (The circuit des faites of the Paris Municipal Commission, see ante page 68).

He says that his formula leads to nearly the same results as have hitherto been adopted, but he gives three examples, the results of which are—length of conductor being 1·00, radius protected is respectively 3·80, 1·10, and 2·20.

In the early part of this paper the author discusses the distribution of electricity in the space between the storm cloud and the earth’s surface, and points out that the air in an electric field is in a state of tension or strain; and this strain increases along the lines of force with the electromotive force producing it until a limit is reached, when a rent or split occurs in the air along the line of least resistance—which is disruptive discharge, or lightning.

Since the resistance which the air or any other dielectric opposes to this breaking strain is thus limited, there must be a certain rate of fall of potential per unit length which corresponds to this resistance. It follows, therefore, that the number of equipotential surfaces per unit length can represent this limit, or rather the stress which leads to disruptive discharge. Hence we can represent this limit by a length. We can produce disruptive discharge either by approaching the electrified surfaces producing the electric field near to each other, or by increasing the quantity of electricity present upon them; for in each case we should increase the electromotive force and close up, as it were, the equipotential surfaces beyond the limit of resistance. Of course this limit of resistance varies with every dielectric; but we are now dealing only with air at ordinary pressures. It appears from the experiments of Drs. Warren de la Rue and Hugo MÜller that the electromotive force determining disruptive discharge in air is about 40,000 volts per centimetre, except for very thin layers of air.

If we take into consideration a flat portion of the earth’s surface, and assume a highly charged thunder-cloud floating at some finite distance above it, they would, together with the air, form an electrified system. There would be an electric field; and if we take a small portion of this system, it would be uniform.

If the cloud gradually approached the earth’s surface, the field would become more intense, the equipotential surfaces would gradually close up, the tension of the air would increase until at last the limit of resistance of the air would be reached; disruptive discharge would take place, with its attendant thunder and lightning.

If the earth-surface be not flat but have a hill or a building, as A or B, upon it, then the lines of force and equipotential planes will be distorted, as shown in fig. 1. If the hill or building be so high as to make the distance HD equal to the limit of resistance (fig. 2), then we shall again have disruptive discharge.

If instead of a hill or building we erect a solid rod of metal, G H, then the field will be distorted as shown in fig. 2. Now it is quite evident that whatever be the relative distance of the cloud and earth, or whatever be the motion of the cloud, there must be a space d d´ along which the lines of force must be longer than c c´ or H D; and hence there must be a circle described around G as a centre which is less subject to disruptive discharge than the space outside the circle; and hence this area may be said to be protected by the rod G H. The same reasoning applies to each equipotential plane; and as each circle diminishes in radius as we ascend, it follows that the rod virtually protects a cone of space whose height is the rod, and whose base is the circle described by the radius G c. It is important to find out what this radius is.

Let us assume that a thunder-cloud is approaching the rod A B (fig. 3) from above, and that it has reached a point D´ where the distance D´ B is equal to the perpendicular height D´ C´. It is evident that if the potential at D´ be increased until the striking-distance be attained, the line of discharge will be along D´ C´ or D´ B, and that the length A C´ is under protection. Now the nearer the point D´ is to D the shorter will be the length A C´ under protection; but the minimum length will be A C, since the cloud would never descend lower than the perpendicular distance D C.

Supposing, however, that the cloud had actually descended to D when the discharge took place. Then the latter would strike to the nearest point; and any point within the circumference of the portion of the circle B C (whose radius is D B) would be at a less distance from D than either the point B or the point C.

“Hence a lightning-rod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle whose radius is equal to the height of the rod.”

Upon this rule the author makes the following concluding remarks:

“I have carefully examined every record of accident that I could examine, and I have not yet found one case where damage was inflicted inside this cone when the building was properly protected. There are many cases where the pinnacles of the same turret of a church have been struck where one has had a rod attached to it; but it is clear that the other pinnacles were outside the cone; and therefore, for protection, each pinnacle should have had its own rod. It is evident also that every prominent point of a building should have its rod, and that the higher the rod the greater is the space protected.”

SHORT ACCOUNT of the STRIKING BY LIGHTNING of the RAILWAY TERMINUS at ANTWERP, on the 10th of JULY, 1865. By M. Melsens, Member of the Royal Academy of Belgium.

On the date mentioned, between three and four o’clock in the afternoon, a violent storm burst over Antwerp, during which the lightning struck the Railway Terminus, without, however, occasioning any other damage than the perforation of a single hole in one of the glass squares of the roof.

The author states that the effect of the discharge on this square of glass, which was about 4^mm (0·2in.) thick, was remarkable; it appeared as if it had been traversed by a projectile from below, the perforation, viewed from above, being broken and chipped, whilst viewed from below it showed a clean edge. The sinuosities caused by the chipping on the upper surface had rounded edges, and the glass appeared to have been subjected to incipient fusion. Not a single fragment of glass was found on the glass squares or in the gutters of the roof.

The author arrives at the following conclusions: The square of glass was pierced in the same manner as any square of similar nature and dimensions, placed in identical circumstances, would be, were it traversed by a spherical projectile fired at a low velocity from a firearm. The fracture resembled one that would be produced by a missile thrown from below, that is to say, from the earth to the sky.

The form of the opening indicated that the earth was positively electrified.

The author notices that, according to M. F. Duprez, negative electricity generally shows itself in abnormal conditions of the atmosphere, during storms, rains, &c., and when the wind blows from the western quarters between N. and S. Now, on the day in question, it rained and the wind blew from the west.

The author publicly thanks M. Ruhmkorff for his skilful and disinterested co-operation in proving the correctness of his (the author’s) view of the distribution of the electricity at the Antwerp discharge. M. Ruhmkorff has, at request, pierced squares of ordinary glass about 1^mm (0·04in.) thick by the discharge of his great induction apparatus charged by a powerful Leyden battery.

ON LIGHTNING PROTECTORS WITH POINTS, CONDUCTORS, and MULTIPLE EARTH CONNECTIONS, a detailed Description of the Lightning Protector erected on the Town Hall of Brussels in 1865, with an Account of the Principles adopted in the Construction, by M. Melsens, Member of the Royal Academy of Sciences of Belgium.

As the author states in his preliminary observations that it is impossible to give a complete condensed description of the Lightning Protector, which he erected on the Town Hall at Brussels, we will merely draw attention to a number of facts, regarding the system followed, some of them, we believe, of a novel description.

M. Daniel Colladon, the author states, has observed that as a rule lightning does not strike a single part or prominent point of the objects that are struck or destroyed by it; and that, in the majority of cases, it does not strike in the form of a single spark, but in the form of a sheet with one or more principal centres of intensity. The correctness of this observation, the author considers fully borne out by the ravages which the electric discharge committed on the Town Hall at Brussels, on the 10th September, 1863. He gives an elaborate description of the effects of the flash on the building. It is interesting to note that the ravages principally took place at the side exposed to the west north-west wind, which was blowing at the time the building was struck.

In the ensuing winter the Municipal Council of Brussels took into consideration the necessity of protecting the Town Hall against a similar disaster, and the author was requested to superintend the erection of lightning protectors on the building.

The characteristics of the author’s system, as exemplified by the lightning protectors erected on the Brussels Town Hall, may be briefly summarised as follows:—

1. The points are very numerous—of three kinds; some long, sharp, and gilded, others of middling length, made of iron; and finally some small and very sharp, consisting of copper.

2. The points are replaced by aigrettes (brushes of points diverging from a common base).

3. The conductor is not insulated.

4. The connections are simple and unchangeable, the joints are each embedded in a mass of zinc.

5. The surface exposed to the air is considerable.

6. The conductor consists of thin, and numerous wires, which are very flexible, so as easily to be led round all the corners of the buildings.

7. The conductor is made of galvanised iron.

8. The earth connections are multiple: firstly, a well within which a large surface of metal is plunged; and, secondly, two enormous networks of metal pipes, offering an immense contact surface with the earth. One of these networks is in direct communication with all the reservoirs and all the water sources of the environs of Brussels and also in indirect communication with two rivers and two canals.

The author has arrived at the conclusion that the height of the rod is a secondary question, as the radius of protection has not been determined by irrefutable proofs, and as that length is, in comparison with the distance and the extent of the thunder-clouds, so small a factor that it may safely be neglected. The author states that he has been greatly gratified to meet with the same opinion in a paper which Mr. W. H. Preece published in Vol. I., No. 3, page 366, of the Journal of the Society of Telegraph Engineers for 1872: “When we consider the distance of the cloud and the area of its surface, the height of a building vanishes in the general figure.”

The author points out that M. Perrot has endeavoured to demonstrate by experiment that the neutralizing area of a lightning protector surmounted by a crown of sharp points is far more extensive than that of an ordinary protector. M Perrott further thought, and MM. Babinet and Gavarret shared his opinion, that it is sufficient to shelter the ordinary protector from discharges of lightning by arming it with numerous, long, sharp, and well conducting divergent points. M. Gavarret after having repeated Mr. Perrott’s experiments, found the results so conclusive that he wrote to the author in the beginning of 1865: “It is at the present time no longer permitted to erect lightning protectors with single points.”

The metal of which the points are made must be a very good conductor. With regard to their conductivity, the metals follow each other in the following order: copper, silver, iron, platinum. No metals are used but those which resist fusion. The author rejected platinum and silver: the former because it fuses very readily by the electric discharge; and the latter, because it has, in his opinion, no advantage over copper.

The conductor, although galvanized, received several coats of paint; but the points (aigrettes) of course remained metallic. With regard to the general principle of connecting the protector with any masses of metal which may be about the building, the author has ever since 1865 endeavoured to demonstrate, that it is not sufficient, as might at first sight be supposed, to form that connection at one single point; there must be at least two points of contact, so as always to ensure a closed metallic circuit.

The contact with the water presents a surface of about ten square metres (12 sq. yds.), bringing both surfaces of the cylinder into account.

With regard to the earth connection, the author quotes M. Perrot, who remarks that with the ordinary protector the surface immerged offers a resistance at least 10,000 times greater than the conductor itself; it is therefore necessary to increase the surface of the earth-plate as much as possible.

In order to retard as much as possible the oxidation of the cylinder, the author introduced two hectolitres (6 bushels) of lime into the well, thus rendering the water alkaline.

DE L’APPLICATION du RHE-ÉLECTROMÈTRE aux PARATONNERRES DES TÉLÉGRAPHES. Par M. Melsens.

In this pamphlet the author describes in § 1 an apparatus to show the presence of atmospheric electricity in telegraph wires.

In §§ 2 and 4 he explains how the apparatus is joined up in the Belgian telegraph offices.

§ 3 contains a rÉsumÉ of observations made at the government telegraph offices between June, 1875, and March, 1876.

The author states in this paragraph that, on the 19th of June, 1875, the Rheo-Electrometer at the office at Louvain, showed a deflection of 85° East, although there was not the slighest appearance of atmospheric electricity. The fact was, that at the time a thunder storm was raging at Beverloo, distant from Louvain about 40 kilometres (25 miles).

TROISIÈME NOTE sur les PARATONNERRES. Par M. Melsens.

On the 3rd of July, 1874, the church of Ste. Croix, at Ixelles, was struck by lightning. The building was provided with a lightning protector, which was constructed as follows: The point consisted of a platinum cone of about 30° (the form officially adopted in France in 1855), all the supports of the protector were soldered with zinc. This was attached to the steeple, and rose to 53 metres or 174 feet above the pavement. It consisted of an iron rod 18 mm. (0·71 in.) in diameter (M. E. SacrÉ’s system). The conductor passed from the principal roof along the roofs, descending to a point near a pump, behind the vestry, where the well (W) was situate. There is an abundance of water in the well, which is about 7 m. (23 ft.) deep. The conductor terminated in the well, by a cast-iron plate 0·65 m. (2 ft. 1 in.) by 0·50 m. (1ft. 8 in.), thus presenting a surface of 0·654 ? m. (7 sq. ft.). A little in front of the transept there is a supplementary rod B 5·25 m. (17 ft. 3 in.) high, 11 m. (36 ft.) distant from the point (c in diagram) which was struck; and 22 m. (72 ft.) distant from that point there was a second rod D, whose height was 9 m. (29½ ft.) above the top of the roof.

The damage to the church was trifling, but the author contends that the fact of the church having been struck at all, proves that a building armed with a protector constructed on the usual principle is not completely protected.

A.	Principal conductor on steeple.
B. D.	Two supplementary receiving rods.
C.	Stone cross at end of transept, which was struck,
W.	Well in which conductor made earth connection.

QUATRIÈME NOTE sur les PARATONNERRES. Par M. Melsens.

This treats § 1 of observations on the distribution of the spark of electric batteries and machines over numerous metallic conductors of different sections, lengths, and nature, and on the passage of electricity of tension in bad conductors.

§ 2. Effects of soldered joints on the conductivity and the resistance of conductors. Interrupted lightning protectors.

§ 3. The distribution of sparks from Holtz’s machine and Ruhmkorff’s coil over two conductors outwardly identical, but one of iron and the other of copper. Comparative resistance to fusion and rupture for iron and copper conductors. Identical damage produced by discharges in several homogeneous and solid conductors.